MICROSCOPES. 25 



and with condensing lens attached by a moveable arm. I have one of 

 these latter instruments, made by Messrs. WILLATS, in my possession, 

 and find it extremely useful for the generality of botanical objects, and 

 indeed for the examination of any specimens where no very great 

 power is required. 



Fig. 9. Fig. 10. 



16. COMPOUND ACHROMATIC MICROSCOPE. In a treatise like 

 the present, it would be utterly impossible for us to enter into any 

 lengthened details concerning the optical principles involved in the 

 construction of these beautiful instruments, or of the means taken by 

 the optician to correct the chromatic and spherical aberration of the 

 lenses. We must again refer such of our readers who desire to become 

 practically acquainted with the subject, to those treatises of which we 

 have already spoken. (P. 18.) 



The annexed engraving (fig. 11) will show the general construction 

 of the form of achromatic microscopes. O is the triple achromatic 

 object-glass. F is a plano-convex lens, called the field-glass, situate 

 in the tube of the microscope, and E E is the eye-glass. These 

 together form the modern microscope. The course of the light 

 is shown by drawing three rays from the centre, and three from 

 each end of the object O. These rays would, if left to themselves, 

 form an image of the object at A A ; but being refracted by the 

 field-glass of F F, they form the image at B B, where a stop is 

 placed to intercept all light, except what is required for the formation 

 of the image. From B B therefore the rays proceed to the eye-glass, 



