motors provided for the purpose. When once directed toward the object to be observed it will frequently happen that the lower end of the telescope is far out of reach above the observer's head. For this reason the

tire floor of the observing room, 75 feet in diameter, is constructed e an electric elevator, which by throwing a switch can be made to se or fall through a distance of twenty-three feet. Thus the lower of the telescope is rendered accessible even for objects near the

horizon. In order that the observing slit may be directed to any part of the sky the dome, 90 feet in diameter, is mounted on wheels and can be turned to any desired position by means of an electric motor controlled from the rising-floor.

The telescope is used for a great variety of purposes in conjunction with appropriate instruments, which are attached to the lower end of the tube near the point where the image is formed. I have already shown a photograph of a star cluster taken with this telescope, but without describing the process of making it. As a matter of fact the object-glass of the 40-inch telescope was designed for visual observations, and its maker, the late Alvan G. Clark, had no idea that it would ever be employed for photography. Without dwelling upon the distinguishing features of visual and photographic lenses I may say that the former is so designed by the optician as to unite into an image those rays of light, particularly the yellow and the green, to which the eye is most sensitive. With the only varieties of optical glass which can be obtained in large pieces it is impossible to unite in a single clearly defined image all of the red, the yellow, the green, the blue, and the violet rays which reach us from a star. Therefore when the optician decides to produce an image most suitable for eye observations he deliberately discards the blue and violet rays, simply because they are less important to the eye than the yellow and green rays. For this reason the image of a star produced by a large refracting telescope is surrounded by a blue halo containing the rays discarded by the optician. These very rays, however, are the ones to which the ordinary photographic plate is most sensitive; hence in a photographic telescope the blue and violet rays are united, while the yellow and green rays are discarded.

The 40-inch telescope is of the first type, constructed primarily for visual observations. In order to adapt it for photography Mr. G. W. Ritchey of the observatory staff simply places before the (isochromatic) plate a thin screen of yellow glass, which cuts out the blue rays, but allows the yellow and green rays to pass. As isochromatic plates are sensitive to yellow and green light there is no difficulty in securing an image with the rays which the object-glass unites into a perfect image. During the entire time of the exposure a star which lies just outside the region to be photographed is observed through an eye-piece magnifying 1,000 diameters. This eye-piece is attached to the frame which carries the photographic plate, and is susceptible of motion in two directions at right angles to each other. In the center of the eye-piece are two very fine cross-hairs of spider web illuminated by a small incandescent lamp. If the observer notices that through some slight irregularity in the motion of the telescope, or through some change of refraction in the earth's atmosphere, the star image is

moving away from the point of intersection of the cross-hairs, he instantly brings it back by means of one or both of the screws. As the plate moves with the eye-piece it is evident that this method furnishes a means of keeping the star images exactly at the same point on the plate throughout the entire exposure. With such apparatus data are gathered for the study of stellar development.

Photographed with the forty-inch Yerkes telescope (Ritchey).

It is easier to trace the successive steps in the development of a star after it has been formed than it is to account for its origin. But all the evidence that has been accumulated up to the present time tends to show that stars are condensed out of the cloudlike masses which we know as nebulæ. Less than half a century has passed since the true nature of a gaseous nebula was determined. In his extensive observations of astronomical phenomena Sir William Herschel examined a great number of star clusters similar to that shown in

Fig. 4. His telescope was a large one, but it can safely be said that he never saw a cluster so well as this object can be perceived through the aid of photography. He found in studying object after object in all parts of the heavens that many clusters could be resolved into their constituent stars. In some of these clusters the stars are widely separated by a powerful instrument, as they appear in this photograph. In others, either on account of their greater distance or because the stars are less widely spaced, the central regions are no longer clearly resolvable as separate objects. It is thus quite possible to imagine a cluster in which the stars are so closely grouped that no telescope, however powerful, could separately distinguish them.

Now as a matter of fact we find in all parts of the heavens luminous objects which can not be separated into stars. Some of these are of definite outline and are perfectly symmetrical in form, in many cases with a brilliant star-like nucleus at their center. These are known as the planetary nebulæ. Other nebulæ, like the great nebula in Orion (Fig. 6), are diffuse and irregular and extend over great regions of the sky. It was long a question whether such objects were.capable of resolution into stars with a sufficiently powerful telescope. Herschel rightly concluded that an important distinction can be drawn between a nebula and a star cluster, though his son did not admit this distinction.

It was only after Huggins had applied the spectroscope to an analysis of the light of a nebula that it could be said without danger of contradiction that the phenomenon is not one produced by the crowding together of separate stars, but is due to the presence of a mass of incandescent gas. Sir William Huggins' account of his first spectroscopic examination of a nebula is recorded in the first volume of the 'Publications of the Tulse Hill Observatory':

"On the evening of August 29, 1864, I directed the spectroscope for the first time to a planetary nebula in Draco. I looked into the spectroscope. No spectrum such as I had expected! A single bright line. only! At first I suspected some displacement of the prism, and that I was looking at a reflection of the illuminated slit from one of its faces. This thought was scarcely more than momentary; then the true interpretation flashed upon me. The light of the nebula was monochromatic and so, unlike any other light I had as yet subjected to prismatic examination, could not be extended out to form a complete spectrum. After passing through the two prisms it remained concentrated into a single bright line, having a width corresponding to the width of the slit, and occupying in the instrument a position at that part of the spectrum to which its light belongs in refrangibility. A little closer looking showed two other bright lines on the side towards the blue, all three lines being separated by intervals relatively dark. The riddle

of the nebulæ was solved. The answer, which had come to us in the light itself, read: Not an aggregation of stars, but a luminous gas."

With this advance a new era of progress began. The power of the spectroscope to distinguish between a glowing gas and a mass of partially condensed vapors like a star established it at once in its place as the chief instrument of the student of stellar evolution. It became apparent that the unformed nebula might furnish the stuff from which stars are made. Observations tending to this conclusion were not long in presenting themselves. In the heart of the Orion nebula are four small stars which constitute the well-known Trapezium. Situated as they are in the midst of this far-reaching mass of gas, it is not hard to picture them as centers of condensation, toward which the play of gravitational forces tends to concentrate the gases of the nebula. It might therefore be expected that stars in this early stage of growth should show through the spectroscopic analysis of their light some evidence of relationship with the surrounding nebula. Now this is precisely what the spectroscope has demonstrated. Not only these stars, but many other stars in the constellation of Orion, are shown by the spectroscope to contain the same gases which constitute the nebula. For this and other reasons they are considered to represent one of the earliest stages of stellar growth.

It may be many years before the exact nature of the process by which a star is formed from a nebulous mass is clearly understood. Shortly before his death the late Professor Keeler made a most important discovery in the course of his photographic work with the Crossley reflector of the Lick Observatory. Spiral nebulæ have long been known, but it was not supposed that they were sufficiently numerous to be regarded as type objects. The great spiral nebula illustrated in Fig. 7 from one of Mr. Ritchey's recent reflector photographs has long been regarded as one of the most remarkable objects in the heavens, and the possible significance of its form had by no means been overlooked. But few astronomers were prepared for Professor Keeler's announcement that the majority of nebulæ are of the spiral form and that many thousands of these objects are within the reach of such an instrument as the Crossley reflector. It does not seem improbable that this spiral form may prove to represent the original condensing mass more truly than the lenticular form from which Laplace imagined the solar system to be evolved.

Enough has already been said to indicate how large a part the methods of spectroscopy must play in a study of the life history of stars. In spite of the common opinion that the spectroscope is an intricate instrument and that the principles of spectroscopy are obscure and difficult of comprehension, it is a fact that the processes used in this field of investigation can be easily understood by any one who will