192 MICROSCOPIC OBJECTS. 



struction, and one at the same time which has no 

 difficulties in its execution. It will therefore be executed 

 rapidly, by ill-paid rubbishing workmen — impose on the 

 pubHc by its outward appearance — and afford a good 

 profit — which is all that is cared for. 



OPTICAL PART. 



In a Megaloscope, I consider that the aim is 

 to obtain a low power, connected with what is called 

 a large field of view. By this I mean not only a field 

 of a great number of degrees of opening, but which 

 also takes in a large portion of the object. Now we shall 

 find in experimenting that there are certain hmits 

 to these qualities, which cannot be passed without detri- 

 ment to others equally important. Thus, suppose we 

 select an object-glass of long focus, in order to obtain a 

 low power ; the length of the body must be proportional 

 (which counteracts the effect of a long anterior conjugate 

 focus) ; moreover the field will not fill with a shaUow 

 eye-glass, unless the body has a certain length, and a 

 deep one neutralizes the effect of the shallow objective 

 part. Again, a shallow eye-glass per se requires a long 

 body, even with a deep object -glass, or it wiU not fill at 

 the edges of the field, which, moreover, wiU be distorted 

 and confused at the margin, and have its central focus 

 much longer than the marginal one. A certain propor- 

 tion between the lengths and size of the body and the 

 foci of its objective and ocular part, being thus always 

 forced upon us, (when at least we wish to have a good 



