THE SPECTRUGKAPII. 



13 



ence to the grating and slit. This was necessary for impressing comparison 

 spectra, etc. H and H' suggest the rack-and-pinion system by the aid of 

 which the lihns could have unexposed portions brought successively opposite 

 to some selected opening in the slide-screen L. D and D' denote two of 



the four doors which gave access to 

 the interior of the spectrograph, and 

 which made it possible to close up 

 the camera light-tight, while making 

 various adjustments with the rest of the 

 system. The camera was made so 

 that, when it contained neither a film 

 nor a plate, it was possible for the 

 experimenter to look directly at the 

 grating and to make observations with 

 the assistance of an ej'e-piece. 



Certain black-on-white scales and 

 ruby-glass windows (Z, for example) 

 enabled the experimenter to know the 

 precise relative positions of the various 

 accessories on the interior of the spec- 

 trograph, when the entire system was 

 shut up and exposures were being made. 

 Numerous dull black diaphragms and 

 screens (A), A2, Ay, Aj, A5, etc.) pro- 

 tected the photographic film from the 

 unusable light which came from the cen- 

 tral image, I, and from all the spectra 

 except the one desired. Ui and Oi 

 give the extreme rays of so much of the 

 first order spectrum as was studied, 

 that is, Ui and Oi correspond respec- 

 tively to about 0.201JL and 0.625^. Ob- 

 vioushs the spectrograph was dull black, 

 both inside and out, and contained 

 plaited black velvet in appropriate 

 places. A general idea of the size of 

 the apparatus may be derived from 

 the following dimensions: From R to 

 the plane of BC = 198.5 cm. ; BC 

 = 34. 5 cm. ; the bottom edge perpen- 

 dicular to BC = 27.5 cm.; BJ=ii6 

 cm. ; and JK = 29 cm. 



Fig. 6. One-lenth natural size. 



