268 Mr. L. Bell on the Absolute 



able; but on the whole the ordinary plan of normal incidence 

 offers fewer experimental difficulties, and therefore was 

 adopted particularly as the spectrometer used was specially 

 well suited to that method. It is quite certain that either 

 method will, with proper care, give the angular deviation with 

 a degree of exactness far surpassing that attainable in the 

 measurement of the gratings. 



The Spectrometer. 



This was a large and solid instrument by Meyerstein, with 

 a circle on silver 32 centim. in diameter divided to tenths of 

 a degree. This is read by two micrometer-microscopes 180° 

 apart. The pitch of the micrometer-screws is such that one 

 turn equals about 2 f ; and as the head is divided into sixty 

 parts, each of these represents 2". The micrometer can, 

 however, be set with certainty to less than half this amount. 

 The collimating and observing telescopes are of 4 centim. 

 clear aperture and 35 centim. focal length, and the lenses are 

 well corrected. The collimator is fixed to the massive arms 

 which carry the reading microscopes ; while the observing 

 telescope is attached to a collar on the axis of the main circle, 

 and moves freely upon it or can be firmly clamped so as to 

 move with the circle. The grating is carried on an adjustable 

 platform with a circle 12*5 centim. in diameter, divided to 

 30', by verniers to 1', and moving either upon or with the 

 large circle. 



This arrangement of parts does not admit of fixing the 

 grating rigidly normal to the collimator ; so in all the experi- 

 ments it was placed normal to the observing telescope, a 

 position which was particularly advantageous in the matter 

 of adjustment. The instrument was set up in a southern 

 room in the physical laboratory, and throughout the experi- 

 ments the collimator pointed about south-south-east. With 

 the eye-piece used, the observing telescope had a power of 

 very nearly sixteen diameters. 



Gratings. 



Very few glass gratings have ever been ruled on Prof. 

 Rowland's engine, since for most purposes they are much 

 inferior to the metallic ones, and are very much more diffi- 

 cult to rule, as they run great risk of being spoiled by the 

 breaking down of the diamond-point. A very few, however, 

 w r ere ruled in 1884 with special reference to wave-length 

 determination ; and of these the two best were available for 



