The next source of light is the electric arc, as formed between electrodes of carbon. This, under good conditions, approaches to a fourth of the brightness of the sun, or about 47,000 times the brightness of a standard candle. Here, then, we have a splendid source of light, and with this great advantage, that the heating rays which accompany it are comparatively few. It is mainly a question of cost and complexity of apparatus. In the present state of electric engineering we should require a gas or steamengine to drive a dynamo, and then either take the light direct from the machine, or make use of a secondary battery in which the electric energy can be stored and used as required. If we adopted the former plan, we should be almost sure to have a very variable light; and if the latter, should run the risk of an entire collapse from the escape of the electric fluid. In either case, we must provide machines and apparatus that are necessarily not only very costly, but very cumbersome. The time may come when electric light shall be supplied as gas is now supplied, and then we may hope successfully to use it in our microscopic lanterns. But, at present in Bath, the electric light is not available, though a constant source of light might be secured by utilising the water which runs to waste over the weir at the old city mills. As a source of light, we next come to the Drummond, or lime-light, but its value is very small as compared with sunlight or electric light. In its most effective form it gives a light equal to about 450 candles, but in general it does not exceed 150 candles. We need hardly mention any of the forms of oil or gas lamps, as few, if any, of them would give us more than a 50-candle power. It appears, then, that for the present we are confined to the use of the lime-light in its best form—that is, to the use of lime, magnesia, or some other earth rendered incandescent by a jet of oxygen and hydrogen gases, mixed before they issue from the blow-pipe. Next, how can this light be most usefully applied so as to produce on the screen a magnified image of the microscopic preparation? It is impossible to transmit to the screen a greater amount of light than falls on the area within which the object is placed. Let us suppose this to be a circle of half-an-inch in diameter, and the screen that we desire to fill nine feet in diameter. It is clear that the light which falls on the half-inch circle will be diluted on the screen to 1-46,656th part of the brightness of the light where it met the object, and if, as must be the case in most minute objects, the area of the enclosing circle does not exceed a quarter or a one-eighth of an inch, the light on the screen will not exceed the 1-186,624th or the 1-746,496th part of the original brightness. It is, therefore, needful that the original brightness of the light, as it meets the object, be increased as the magnifying power is increased. The rays of light that proceed from the incandescent lime are divergent, passing forward in straight lines in every direction, and only such portion of them as we can compel to change their direction are of any use to us. The first thing, therefore, is to convert these diverging rays into a parallel beam of light. This may be effected by means of a concave mirror or a convex lens. For the purpose we have in hand, a lens will be the more convenient. The action of a second lens changes this parallel beam into a convergent beam, and in this convergent beam the object is placed, its position being dependent on its diameter. The final result on the screen will depend to a great extent on the accuracy with which this conversion of the rays is accomplished. The source of light must be concentric with the optical axis of the lenses, and the lenses must be of such a character, both as regards their transmitting and their refracting powers, as to arrest as little light as possible, and conduct it forward with the least possible dispersion. Theoretically, there ought to be no difficulty in bringing the light which issues from the lime into a cone with a very fine apex; but in practice it is very difficult. There is a large amount of light reflected from the various surfaces of the lenses the light itself is not a simple point-and several other causes contribute to make the cone of rays very imperfect. And yet the adequate illumination of the smaller objects depends almost entirely on the perfection of this cone. An arrangement should be made whereby the object to be illuminated can be placed in that part of the cone of light which will best cover it. Thus, the more minute the object, the nearer the apex, and vice-versa. If the cone were perfect, the screen ought to be equally well lighted under all conditions of amplitude; but this is not found to be the case. The objects that require least amplification are always shown on the screen brighter than the more minute. While we are considering that part of the arrangement that concerns the condensation of the light, there is a point which requires attention. The amount of heat produced by the combustion of the hydrogen and emitted from the incandescent lime is so great that when collected by the lenses it would spoil any delicate preparation, unless some means were adopted to absorb these heating rays. This is best effected by interposing a cell with parallel glass sides containing a saturated solution of common alum (potassium and aluminium sulphate); but the use of this solution has been claimed as a special point in the patent of Messrs. Wright and Newton, and so not to infringe their patent right we can dispense with the alum and use the water only. This will intercept by far the greater part of the heating rays, and those which do pass will not produce any inconvenience. The remaining arrangements are those of the ordinary microscope, except that we dispense with the eye-piece, taking our image to the screen direct from the object-glass. We also dispense with the plane or concave mirror, usually placed beneath the stage of the compound microscope. In displaying objects on the screen, the effect is much more pleasing if the margin of the field is well defined. There are considerable difficulties in obtaining this with lenses of high powers. This is because the object to be shown is seldom or never in the same plane as the margin of the diaphragm, and it is this which limits the illumination on the Even when the object is so placed that only the cover glass interposes between the object and the stage, the thickness of this is sufficient to give a disc with blurred edges. This might, perhaps, be obviated by placing a stop between the object-glass and the screen. screen. There is another point of great importance as affecting the result, and that is the suppression of all light except that which passes through the object-lens. The lantern should be so constructed as not to allow any light to pass into the room, and all parts where reflectors or refractors are employed should be carefully guarded, and when it is possible the walls and ceiling of the VOL. VI. L 146 THE MICROSCOPE IN THE LECTURE AND CLASS-ROOM. room should be of a dark colour. Especially is the proximity of a white ceiling to be avoided. Light is reflected from the screen to such a ceiling, and from it again to the screen, and a great loss of effect is produced. The object should be projected on and not passed through the screen. A good, smooth, plastered wall forms the best screen : calico, faced with smooth, white paper, comes next. If the conditions are such as to make a translucent screen a necessity, tracing-cloth or tracing-paper will be found to act much more satisfactorily than a wetted sheet. In all cases where a translucent screen is used, all spectators that are near the axial line of the light and lenses see a bright spot of light in the centre of this field of view that greatly detracts from the effect. But when we have done our best and obtained the greatest enlargement and definition at present practicable, what does it amount to? Will it enable the lecturer to place before his audience all that he requires to illustrate his subject? Will it place within the reach of the teacher the power to demonstrate to his whole class, at one and the same time, all the special points that call for particular notice? I am inclined to think that for the lecture-room good photo-micrographs will be found more useful than real objects shown direct on the screen. The lecturer has not often occasion to enter into many of those details of ultimate structure that are so important to the teacher, and a good photomicrograph, prepared beforehand, and specially arranged to illustrate the precise point of the lecture, will be more effective when produced before a large audience than a real object when shown on the screen. At the same time, the manipulation of the ordinary lantern is much simpler. To the teacher, I think that the oxy-hydrogen microscope, in its best form, offers more advantages. In the first place, it will not be needful to employ so large a screen, and therefore the light will not be so greatly diffused, and the resulting picture will be brighter. Then the ordinary arrangements of a class admit of the more close examination of the image on the screen, and many details that could not have been seen by a large company would be readily seen by a few students, who could closely examine the image on the screen. Still, I have no doubt that there will be many points in the ultimate tissues, both of animals and vegetables, that the oxy-hydrogen microscope will fail to display, and for which no substitute for the direct eye of the observer can be provided. POSTSCRIPT. At a subsequent meeting of the Society, held at his own house, Mr. Pumphrey conducted a series of experimental demonstrations, showing the advantages to be derived from the exhibition on the screen of magnified images of actual objects, and also pointing out the difficulties attending the process, and where it was most likely to fail. Carefully-prepared micro-photographs were displayed on the screen, along with magnified images taken direct from the same objects; and the opinions advanced in Mr. Pumphrey's paper were endorsed by all present—namely, that when the object was to demonstrate to a class, or to a small company, who can critically examine the image as displayed on the screen, the image, as taken direct from the object, was much to be preferred; but that for large companies, and where the close examination of the image would be impracticable, the micro-photograph was better adapted to the purpose. The subject was also considered with reference to the attainment of the same end by means of carefully-prepared diagrams and black-board illustrations. It was felt that unless there was a probability of the former being frequently used, the requisite expenditure of time and labour was too great for most persons; but as regards the latter the opinion was expressed that the free use of the blackboard, in illustration of the papers read to the Society, was a great advantage. On the Water in the Chalk beneath the London Clay in the London Basin.* MY By ROBERT B. HAYWARD, M.A., F.R.S. Y object in the present paper is not to bring forward any new facts or observations, but the collocation and juxta * Read at a meeting of the County of Middlesex Natural History Society. |