352 



SUMMAKY OF CURRENT RESEARCHES RELATING TO 



of the collimator tube to the projection tube. This inconvenience is 

 avoided by affixing the collodion grating to one of the faces of a glass 

 prism. The glass used is boro-silicate-crown, 144 of Schott's 

 catalogue, with a refractive angle of 38° 56'. The rays proceeding from 

 the collimator tube fall perpendicularly on the base AB of the right- 

 angled prism ABC, fig. Gl, and when refracted through the hypotenuse 

 A C are simultaneously dispersed by the grating which is affixed to A C. 

 [The angle B A C is 38° 56'.] The red end of the first diffraction 



Fig. 62. 



spectrum is towards the angle A, and the violet end towards C. A ray 

 of medium wave-length (of perhaps 0*56 fj.) passes through undeviated. 

 Fig. 62 gives in approximately natural size a sectional view of the 

 apparatus. S/> is the entrance slit whose width is regulated by a spindle 

 provided with a left and a right-handed screw, and the graduations on 

 the drum J give the width of the slit in hundredths of a millimeter. 

 C is an achromatic collimator objective of 32 mm. focus and 6 mm. free 

 aperture. Above this is a polariser N" of Aureus' construction, detach- 

 able by a lever K, thus allowing the use of the instrument as a spectro- 



