114 



MICROSCOPIC ILLUSTRATIONS. 



indeed, prosecuted the investigation of this subject by 

 means of the microscope, to a much greater extent than 

 any one else, by making it available towards exhibiting 

 the rings in plates of crystals cut across their axes, and 

 the exquisite tints in unannealed glass, &c. 



Fig. 25. 



In these experiments I find that achromatic object- 

 glasses are almost essential ; for with common ones the 

 aperture is necessarily so reduced, that a sufficient 

 quantity of light will not be transmitted to give 

 intensity to the colouring. The object-glasses classed 

 under A, will be found most suitable ; and the eye-piece, 

 J5, best constructed to take in the largest field admissible 

 by the prism. 



Let iy fig. 25, represent the body of a compound micro- 

 scope, carefully placed over the centre of the spring- 

 double-stage, , b. Let a slider of crystals be placed 

 upon the. stage, a, b ; adjust to the focus, and illuminate 



