INTRODUCTORY REMARKS. 



39 



Suppose that we are going to look at some spot on the 

 surface of a, or a, which we will imagine to be a delicate 



Fig. 1. 



Fig. 2. 



tissue. By a well-known law of light, the rays proceed 

 in right lines, in all directions, from this spot, in the 

 manner shewn by the dotted lines in both figures. 

 Suppose b, b, and b, b, to be two object-glasses, of equal 

 focal lengths ; the former a single lens, of the best 

 construction, such as was used in the old compound 

 microscope, and the latter a lens of the newest form, 

 termed an achromatic. Now, these object-glasses will 

 form their respective images at i, and i, and they will be 

 of equal dimensions. But if the number of rays pro- 

 ceeding from a, and falling upon the single lens b, h, is 



