PHYSIOLOGY OF THE INVERTEBRATA. 



171 



edges. This opens so widely as to permit a view of the 

 whole visual field. The slit is shortened by the screw H, so 

 that when the comparison prism is inserted the aperture is 

 contracted to such an extent that the image of the object 

 under investigation completely fills it, .There is a comparison 

 prism, with lateral frame and clips to hold the compared 

 object and the mirror; all these parts are fixed in a drum 

 combined with the eyepiece. Above the eyepiece there is 



Fig. 32. — The Abbe-Zeiss Mickospecteoscope. 



I = Microspectroscope (half actual size). 2 = Drum with mechanism 

 of the slit (actual size). 



an Amici prism of great dispersion, which turns aside on a 

 pivot, leaving the eyepiece unobstructed for adjustment to 

 an object K; the axial position of the prism is indicated 

 by the spring L, which keeps it in place. A scale is projected 

 on the spectrum by means of a small scale-tube and mirror 

 attached to the mount of the prism. The divisions of the 

 scale give the wave-lengths of that section of the spectrum 

 on which they fall in fractions of a micromillimetre, whereby 

 the second deqimal place may be read oflF directly, and the 



