PROPERTIES OF LENSES. 



23 



For low powers and the majority of pathological 

 investigations very large angular aperture is not 

 considered advantageous, as with every increase of it 

 the penetration and working distance decreases. 



We hope that these explanations will enable the 

 reader to choose an objective suitable for the parti- 

 cular branch of study he may be engaged in. 



The extra amount expended in the purchase of 

 first-class objectives, will produce a considerable 

 saving in subsequent expenses, on account of the 

 increase of successful results due to their employ- 

 ment. We therefore strongly recommend the ama- 

 teur to patronise only the best makers, and pay the 

 highest price for his objectives. 



We shall devote a little time to the consideration 

 of the principles involved in the formation of the 

 image in the compound microscope, although we are 

 not directly concerned with it in microphotography. 



The compound microscope in its simplest form is 

 composed of two lenses the objective and eye lens ; 

 fig. 13, 1 and 2, it will be perceived that the real 



image a b of the object A B formed by the objective 

 at a point just within the focus of the eye-glass is 



