ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 



823 



(a) A cylindrical lens F, for lengthening out the spectrum. 



(b) A small right-angle prism G, sliding in and out of the 

 field of view, which, when slid in, projects over half the field and 

 throws an image of the dark bands in a piece of quartz polarized 

 by means of two Herapathites or flakes of iodide of disulphate of 

 quinine, and termed 



(c) The " Standard Scale," H. This portion of the appa- 

 ratus is used when the observer desires to record the position 

 of the absorption bands. The plate of quartz vrhich it con- 

 tains is cut parallel to the optic axis,* of such a thickness that 



Fig. 184. 



the line D in the solar spectrum comes between the third and 

 fourth band. It is thus described by Mr. Sorby : — " In order 

 to measure the exact position of absorption bands, &c., seen in 

 spectra, I have contrived a small apparatus which gives an inter- 

 ference spectrum divided by black bands into twelve parts all of 

 equal optical value. It is composed of two Nicol's prisms, 

 or Herapathites, with an intervening plate of quartz about 

 043 inch thick, cut parallel to the principal axis of the 

 crystal, the thickness being so adjusted that the sodium or D 

 line is exactly 3^, counting the bands from the red end towards 

 the blue." 



4th. A small lens I, to condense the light from the object. And 

 5th. A very accurate slit K, one side of which is adjusted by 

 means of the milled head L. 



Mode of Use. — Screw the object-glass A into the body of the 

 Microscope ; slide the rest of the apparatus on, and turn it round 



* It is well known that a plate of quartz cut parallel to the optic axis will, 

 under polarized light, give a series of black bands, the distance between such 

 bands being due to the thickness of the plate of quartz. 



