254 Mr. Louis Bell on the Absolute 



the most generally useful, while the third is the most objec- 

 tionable. The method of minimum deviation slightly increases 

 the experimental difficulties, but often improves the definition 

 of the gratings, and is capable of giving very exact results. 

 The last method is applicable only when the spectrometer is 

 so rigid as to ensure the permanence of the angle between 

 the telescopes. When this condition is fulfilled, however, the 

 method is very valuable, since it reduces the moving mass to 

 a minimum and allows the method of repetition to be readily 

 used. 



In the present research, for the work with glass gratings, 

 the second method was selected as best suited to the arrange- 

 ment of the spectrometer. This was a very good instrument 

 by Meyerstein. The circle is '32 centim. in diameter, divided 

 on silver to ()', and reading by two microscopes directly to 2" 

 and by estimation easily to within 1". The collimating- and 

 observing-telescopes are of 4 centim. clear aperture and about 

 35 centim. focal length, well corrected and firmly supported. 



For the second part of the work, with speculum-metal 

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

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

 described instrument, both by reason of the small aperture of 

 the telescope and the inability of the grating-holder to carry 

 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 has 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 telescopes are of 16 - 4 centim. clear aperture and about 

 2"5 metres 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 metres, and bears at the extremity 

 of the shaft the spectrometer proper. This is an instrument 

 by Schmidt and Haensch, having a circle 32 centim. 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 appre- 

 ciable amount. The circle, however, was not finely em nigh 

 graduated, nor were the microscopes of sufficient power to 



