274 L. Bell — Absolute Wave-length of Light. 



gratings, it was desirable to use gratings of the largest size 

 practicable, far larger than could be used on the above de- 

 scribed instrument, both by reason of the small aperture of 

 the telescope and the inability of the grating holder to cany 

 the requisite mass steadily. This part of the work was, there- 

 fore, carried out on a very large instrument, designed by Prof. 

 Rowland especially for using gratings of the largest sizes as 

 yet ruled. This instrument lias virtually fixed telescopes, 

 solidly clamped, with a small lateral range of adjustment, to a 

 T-shaped casting bedded in cement which in turn forms the 

 top of a large brick pier resting on a stone slab. 



The telesco23es are of 16'4: cm clear aperture, and about 2'5 

 meters focal length and the objectives are of excellent quality. 

 Each telescope is fastened to an arm of the T, which has a 

 total length of over 2 meters, and bears, at the extremity of the 

 shaft the spectrometer proper. This is an in strum ent by Schmidt 

 and Haensch, having a circle 32 cm in diameter divided to 6' 

 and, as in the other spectrometer, reading by two microscopes 

 directly to 2" and by estimation to less than half that amount. 

 The original central platform had been removed and replaced 

 by a grating holder large enough to carry if necessary a 6 inch 

 grating. Such an apparatus limits one, of course, to the fifth 

 method, but so rigid is the whole affair that experience soon 

 showed that the angle between the telescopes did not change 

 by any appreciable amount. The circle, however, was not 

 finely enough graduated, nor were the microscopes of sufficient 

 power to derive the fullest benefit from the size of the tele- 

 scopes ; over and over again has the fine in the spectrum ap- 

 peared slightly displaced from the crosshairs, when no differ- 

 ence whatever could be detected in the micrometer readings. 

 However, there was gained the great advantage of using grat- 

 ings of a decimeter in length, giving spectra of great bril- 

 liancy and superb definition, and which could be measured 

 with vastly greater exactness than is possible with the small 

 gratings generally employed. 



Gratings. 



Four gratings have been used in my experiments — two of 

 glass and two of speculum metal. The former are probably 

 the best of the very few glass gratings that have been ruled on 

 Prof. Rowland's engine. They are ruled on plane sextant 

 mirrors of rather hard glass. 



Grating I, contains 12,100 spaces in a length of very nearly 

 thirty millimeters, the lines being nineteen millimeters long. 

 It was ruled in Jan., 1884, at a temperature of 6° - T C. gives 

 spectra of excellent definition, quite free from ghosts or false 

 lines, and having almost exactly the same focus on both sides 

 of the normal. 



