102 The Microscope. 



in the construction of the Microspectroscope, hence a "direct 

 vision " prism is commonly used. This is a compound prism 

 consisting of three or more simple prisms as shown in Fig. 2. 



Two of these are made of crown glass, which, while having the 

 same refraction as the two crown glass prisms, greatly exceeds 

 them in dispersive power. Thus it is, that while the ray of light 

 is dispersed by passing through this form of prism, it is not re- 

 fracted—that is, the middle of the spectrum (green) is in a 

 straight line with the incident beam. A prism of too great dis- 

 persion should not be used with the Microspectroscope for rea- 

 sons which will be given further on. 



The construction of the microspectroscope is similar to that 

 of a two-inch eye-piece, with the slit placed in the focus of the 

 eye-lens, the latter being achromatic and adjustable for focus 

 upon the slit. Above the eye-lens is placed the compound 

 prism. The slit is made adjustable for width and length and 

 in the more complete forms of the instrument a small right- 

 angled reflecting prism may be drawn over half of the slit so 

 that light may be reflected from some lateral source through it. 

 In such arrangements the side of the instrument is provided 

 with a small stage and mirror ; the former for holding any ma- 

 terial whose spectrum is to be compared with the object on the 

 stage, and the latter to reflect the light through that object. 



Fi»-. 3 will give a very good idea of the external appearance 

 of the spectroscopic eye-piece as made by R. tfc J. Beck, which 

 is the form I use. 



The compound prism is contained in the tube M, which also 

 carries the achromatic eye-lens adjustable by the large milled 

 head. At H is shown the milled head for the adjustment of the 

 width of the slit, while the lever L adjusts its length. The 

 milled head I, is for bringing the comparison prism down over 

 half of the slit, so that the spectra of objects upon the stage E, 

 may be compared with those upon the stage of the microscope. 

 For the convenient holding of bottles or tubes containing solu- 



