Oe pees hs aed 
oe 
310 J. P. Cooke on the Spectroscope. 
tion, moving of course the prism on the plate without disturb- 
ing the position of the plate itself, In like manner other prisms 
may be added until the required number is obtained. (In the 
Cambridge spectroscope there are nine glass prisms of 45°, as 
shown in fig. 4.) This adjustment has only to be made, how- 
ever, once for all, since 
when the prisms are Fig. 4. 
adjusted they are fast- 
ened to a very thin, 
screws. The ends of 
are attached to a small 
ete 
= 
3 
pose 
Ss 
n up by an 
ordinary clock spring, 
raws the chain 
keeps them always in 
place. By tracing the 
it will be found Seas : 
‘the path of the ray within the prisms is always tangent to 
the same circle, provided that it passes through all under the 
- conditions of least deviation. Assuming, then, that the dis- 
nees between the prisms are invariable, as they must be when 
, ae risms are fastened to a brass ribbon as just desonibets 
wi 
_ +he dimensions of the conicai wheel A, fig. 2, were thus deter- 
‘mined—the largest diameter, 94 inches, corresponding to the 
- lest diameter, 83 inches corresponding 
he solar spectrum. In order to 
series of lines were engraved encil- 
