172 



PHYSIOLOGY OF THE INVERTEBRATA. 



third calculated by estimation. The position of the scale 

 relative to the spectrum is adjusted by a screw P on the 

 jacket of the Amici prism. 



Pig. 33 represents Dr. Engelmann's microspectrometer, 

 which is constructed on the principle of Vierordt's spectro- 

 photometer for quantitative microspectrum analysis. In 

 place of the eyepiece, the box A is attached to the body of 



Fig. 33. — The Engelmann Microspectrometer. 



the microscope by the tube E ; it contains two independent, 

 conaxial, movable slits in juxtaposition, which are sym- 

 metrically opened and closed by opposed reverse-threaded 

 screws. The width of each slit is read off on the drums T 

 and T' accurately to 0"0i mm., and by estimation to 

 O'OOi mm. One slit is occupied by the image of the object 

 under investigation, and the other by light. from the source 



