CONDITIONS OP EFFICIENCY. 



The object which is submitted to our sense of vision with the 

 microscope, being the optical image produced by the object-glass, 

 our perception of it can only be clear and distinct, provided the 

 three conditions above stated are fulfilled, that is, provided it be 

 viewed under a sufficient visual angle, that its outline and 

 lineaments be shown with perfect distinctness, and that it be 

 sufficiently illuminated. 



The conditions, therefore, upon which the efficiency of the 

 microscope must depend will necessarily be those which will 

 confer upon the image, submitted to the observer, the qualities 

 above stated. 



The optical conditions which determine the visual angle or 

 apparent magnitude of the image, as viewed with the eye-glass, 

 have been already explained ; and it is evident that these con- 

 ditions can always be fulfilled, provided object-glasses and eye- 

 glasses of sufficiently short focus can be produced. Speaking 

 generally, the magnifying power of the object-glass will be limited 

 by the proportion which the length of the microscope will bear to 

 its focal length ; and the magnifying power of the eye-glass will 

 be limited only by its power of approaching sufficiently close to 

 the image, without too much contracting the field of view. 



If the purpose of the observer were merely to see the object as 

 a whole, so as to obtain a perfectly accurate notion of its outlines, 

 a moderate magnifying power would, in general, suffice. But in 

 most microscopic researches it is desired to ascertain, not merely 

 the general outlines of the object, but the far more minute linea- 

 ments of its structure ; and to render these visible in the minuter 

 class of objects, amplifying powers of a very high order are 

 indispensable. 



9. The powers, indeed, which exhibit to the observer the 

 general outline of an object, are rarely sufficient to show the 

 minute lineaments of its structure. To perceive the general 

 outline, it is necessary that the entire object should be included 

 at once within the field of view, and this could not be the case, 

 if the magnifying power exceeded a moderate limit. The power, 

 on the other hand, which is sufficiently great to show the most 

 minute parts of the structure, would necessarily be so great that 

 a very small part only of the entire object would be comprised in 

 the field of view. 



From these circumstances it will be readily understood, that 

 each class of powers have their peculiar uses, neither superseding 

 the other ; when we desire to observe the general form of a micro- 

 scopic object, we must view it with a low power. When we 

 desire, on the other hand, to examine its parts, and if, for example, 

 it be an animalcule, to observe it member by member, and organ 



