ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 



957 



glass screen lighted with a small mirror, and a set of revolving 

 diaphragms to adjust the amount of light. 



At first it will seem strange to have a light before the eye not in 

 use, but after a short time this will wear off, and it will then be 

 found that far brighter illumination of the field can be borne when 

 using this device than when closing one eye or employing a black 

 screen. 



Abbe's Micro-spectroscope.— This was described at p. 703 of 

 Vul._ III. (1880), with an outline diagram of its construction. Its 

 special feature consists, it will be remembered, in the arrangement by 

 which the position of the lines in the spectrum is determined by a 

 direct reading of their wave-lengths on a scale in fractions of //,. The 

 apparatus (half natural size) is shown in fig. 149, and the arrangement 

 for widening the slit in fig. 150. 



The tube J containing the prism moves on the excentric pin K 

 so as to turn it away from the eye-piece when required for focusing 

 the object. It is fixed in place by the catch L. The slit is in the 



Fig. 149. 



Fig. 150. 



drum A, and is made wider or narrower by the action of F which 

 causes the plates B and C, connected by the lever-arm G and movincr 

 between the guides D and E, to approacli each other symmetrically" 

 H, on the other hand, regulates the length of the slit. The scale N 

 (fig. 101) is illuminated by the mirror O, and its image is thrown on 

 the spectrum by the objective at R. By tlie milled head P wbich 

 acts against the spring Q, it is set so tliat the Prauuhofer 'line D 

 coincides with • 581) of the scale. The screw M serves to secure the 



