ZOOLOGY AND BOTANY, .MICROSCOPY, ETC. 



361 



any line can thus be registered. The gradations of the revolving drum 

 are observed through the lens by a slight shift of the eye, and in this way 



Fig. 53. 



the positions of numbers of lines can be rapidly recorded. The whole 

 spectrum is divided into 50i) divisions, and a glance at the sun at once 

 gives the ratio that the scale bears to the actual wave-length. 



Fig. 54. 



Beck's Large Model Wave-length Diffraction Spectroscope (fig. 54). 

 This instrument consists of a large collimator, a diffraction grating, and 

 June 19th, 1907 2 b 



