TEST OBJECTS. 



113 



The angles B, A, B, and b, a, b, are the angles of aperture 

 of the respective object-glasses, and the quantity of light 

 transmitted will be as the squares of B, B, and b } 6, their focal 

 length being equal. 



Fig. 30. 



Fig. 31. 



It may be supposed, that if we throw more light upon an 

 object, so that more may be collected by the object-glass, we 

 shall be better able to define its structure ; and this would 

 probably be the case if we could throw light only upon those 

 minute parts which we wish to examine, and not upon the 

 whole object, but as we cannot increase the relative propor- 

 tions of light, the advantages proposed cannot be derived. 



10* 



