Page images
PDF
EPUB

being purified from barium by a tedious process of fractional | unknown line by the original formula, and then calculate crystallisation of the bromides is almost certain to show barium lines in its spectrum. I have only been able to use about a grain of perfectly pure radium bromide, all other samples containing traces of barium.

Owing to the length of the spectrum and the necessity of having the lines near the position of minimum deviation to get the greatest sharpness, each photograph is limited to a small extent of spectrum, and eight exposures are needed to take in the whole ultra-violet spectrum, and as far into the visible part as the plates are sensitive to.

The accompanying table gives the wave-lengths of all the lines ascribed to radium by different observers. Many of the early observations are necessarily imperfect, owing to the enormous difficulty of preparing radium compounds of sufficient strength to show a photographed spectrum. And when sufficient concentration was obtainable, the observations were necessarily limited owing to the minute amount available rendering verification difficult.

B

the approximate wave-length of the third known line by the same formula, as if it were unknown, using the two original standards for this purpose also. We have now the approximate wave-length of a known line, as given by the formula, and also its true wave-length. The difference between these two values leads (see below) to the correction to apply to the approximate value of the unknown line. It often happens that four or more lines are well placed for use as standards. If any three of these be taken, and from them the positions of the other lines be calculated, it will sometimes be found that there is a small residual error in some of them. In such a case the error can be minimised by adjusting the value of the three primary standards so that the sum of the errors on all the lines is a minimum, and in no individual case is very great. Then, with these three corrected standards, the unknown lines may be cal culated with confidence to seven figures.

With few exceptions my standards are the most recent published by Rowland. He has given two sets of" Standard Wave-lengths," one in "Astronomy and Astrophysics," vol. xii., p. 321, published in 1893; the other in the Astrophysical Journal, vol. i., No. 1, January, 1895, to vol. v., No. 3, March, 1897, and vol. vi., No. 5, December, 1897. Any systematic or accidental error found to occur in Rowland's figures will require a corresponding correction of my own wave-lengths. Assuming Rowland's wavelengths to be correct, I can follow his seven-figure standards with a probable error of 0002 at the most refrangible end, and one of oor at the least refrangible end. The average error being +0.005. But on the assumption that all Rowland's measurements are incorrect by a variable amount, as rendered probable by the recent work of Fabry and Perot (Comptes Rendus, May 28, 1901, vol. cxxxii., No. 21), a correction will have to be made which will affect the sixth figure.*

For the reduction of the lines 4682 149 and 4825.896, there being no well-defined iron lines suitable for measurement, I have used some strong zinc and cadmium lines, which also have been measured by Rowland.

The following is the method of calculation I now employ :

NI λι 112 12

FIG. 2.

Measurements are taken of the exact distances apart of the radium lines and certain adjacent iron lines used as standards. By using a formula, first communicated to the British Association by Sir George Stokes, the wavelengths of the unknown lines can be calculated. At the time the formula was given it was sufficiently accurate for the instrumental means employed for photographing spectra, but the formula only gave approximate results, and the accuracy of determination of wave-lengths has since improved so much that a correction is required to the original formula. Sir George Stokes, before whom I placed the difficulty in June, 1895, quickly solved it in a satisfactory manner. The usual formula requires the positions of two standard lines of known wave lengths, n, and n3, on each side of the unknown line, n2. To make the small correction, Sir George advised me to take a third line of known wave-length, chosen well removed from the selected known lines ni and nз. If chosen in the interval 1-3 it had better not be greatly distant from the middle. There is, however, very wide latitude of choice in this respect. All these lines must be photographed and measured in the usual way. Calculate the approximate wave-length of the

[ocr errors]

n1 and #3 are scale positions on the measuring machine of standard lines of known wave-lengths, Ar and A3. n2 is the scale position of the line whose wave-length is required (A2).

14 is the scale position of an additional standard line whose wave-length is known. It is used for obtaining the correction to apply to the approximate wave-length of λ2.

E2 and E4 are the calculated errors obtained for 2 and A4, which have to be added to or subtracted from A2 or 4 to get them accurate.

For the tedious calculations involved in the reduction of the wave-lengths I am indebted to my son, Mr. Bernard H. Crookes, M.Sc.

Rule.

[merged small][merged small][merged small][merged small][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][ocr errors][subsumed][ocr errors][merged small][merged small][merged small][merged small]
[blocks in formation]

SUPPLEMENT TO THE CHEMICAL NEWS, OCTOBER 23rd, 1903.

2831.67

[blocks in formation]

Plate 1.

[graphic]
[graphic]

Owing to the length of the spectrum and the necessity of having the lines near the position of minimum deviation to get the greatest sharpness, each photograph is limited to a small extent of spectrum, and eight exposures are needed to take in the whole ultra-violet spectrum, and as far into the visible part as the plates are sensitive to.

being purified from barium by a tedious process of fractional | unknown line by the original formula, and then calculate crystallisation of the bromides is almost certain to show the approximate wave-length of the third known line by the barium lines in its spectrum. I have only been able to use same formula, as if it were unknown, using the two original about a grain of perfectly pure radium bromide, all other standards for this purpose also. We have now the apsamples containing traces of barium. proximate wave-length of a known line, as given by the formula, and also its true wave-length. The difference between these two values leads (see below) to the correction to apply to the approximate value of the unknown line. It often happens that four or more lines are well placed for use as standards. If any three of these be taken, and from them the positions of the other lines be calculated, it will sometimes be found that there is a small residual error in some of them. In such a case the error can be minimised by adjusting the value of the three primary standards so that the sum of the errors on all the lines is a minimum, and in no individual case is very great. Then, with these three corrected standards, the unknown lines may be calculated with confidence to seven figures.

The accompanying table gives the wave-lengths of all the lines ascribed to radium by different observers. Many of the early observations are necessarily imperfect, owing to the enormous difficulty of preparing radium compounds of sufficient strength to show a photographed spectrum. And when sufficient concentration was obtainable, the observations were necessarily limited owing to the minute amount available rendering verification difficult.

B

in

With few exceptions my standards are the most recent published by Rowland. He has given two sets of" Standard Wave-lengths," one in "Astronomy and Astrophysics," vol. xii., p. 321, published in 1893; the other in the Astrophysical Journal, vol. i., No. 1, January, 1895, to vol. v., No. 3, March, 1897, and vol. vi., No. 5, December, 1897. Any systematic or accidental error found to occur Rowland's figures will require a corresponding correction of my own wave-lengths. Assuming Rowland's wavelengths to be correct, I can follow his seven-figure standards with a probable error of 0.002 at the most refrangible end, and one of o‘or at the least refrangible end. The average error being +0.005. But on the assumption that all Rowland's measurements are incorrect by a variable amount, as rendered probable by the recent work of Fabry and Perot (Comptes Rendus, May 28, 1901, vol. cxxxii., No. 21), a correction will have to be made which will affect the sixth figure."

For the reduction of the lines 4682 149 and 4825.896, there being no well-defined iron lines suitable for measurement, I have used some strong zinc and cadmium lines, which also have been measured by Rowland.

The following is the method of calculation I now employ :

[merged small][ocr errors][ocr errors][merged small]

FIG. 2.

Measurements are taken of the exact distances apart of the radium lines and certain adjacent iron lines used as standards. By using a formula, first communicated to the British Association by Sir George Stokes, the wavelengths of the unknown lines can be calculated. At the time the formula was given it was sufficiently accurate for the instrumental means employed for photographing spectra, but the formula only gave approximate results, and the accuracy of determination of wave-lengths has since improved so much that a correction is required to the original formula. Sir George Stokes, before whom I placed the difficulty in June, 1895, quickly solved it in a satis factory manner. The usual formula requires the positions of two standard lines of known wave lengths, n, and n3, on each side of the unknown line, n2. To make the small correction, Sir George advised me to take a third line of known wave-length, chosen well removed from the selected known lines ni and nз. If chosen in the interval 1-3 it had better not be greatly distant from the middle. There is, however, very wide latitude of choice in this respect. All these lines must be photographed and measured in the usual way. Calculate the approximate wave-length of the

#1 and #3 are scale positions on the measuring machine of standard lines of known wave-lengths, Ar and λ3. n2 is the scale position of the line whose wave-length is required (A2).

14 is the scale position of an additional standard line
whose wave-length is known. It is used for ob-
taining the correction to apply to the approximate
wave-length of λ2.

E2 and E4 are the calculated errors obtained for A2 and
A4, which have to be added to or subtracted from
A2 or 4 to get them accurate.

For the tedious calculations involved in the reduction of the wave-lengths I am indebted to my son, Mr. Bernard H. Crookes, M.Sc.

Rule.

[merged small][subsumed][ocr errors][merged small][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small][ocr errors][merged small][merged small][merged small]
[blocks in formation]

SUPPLEMENT TO THE CHEMICAL NEWS, OCTOBER 23rd, 1903.

[blocks in formation]

2807'10

2707 23

[blocks in formation]

Plate 1.

[graphic]
[graphic]
« EelmineJätka »