1895 



THE MICROSCOPE. 



169 



tripod, which costs but a trifle and as these are Eiade con- 

 taining two lenses, both double convex, one thicker and 

 one thinner, they do excellent service. The lenses in the 

 trij)od are one and a quarter inches in diameter but are 

 diaphragmed down to four or five eighths. The focal dis- 

 tance of the triplet and tripod may be the same and for 

 the purpose of dissecting a one-inch focus is very useful as 

 this is far enough from the working stage to allow of the 

 free use of needles. One may estimate the focal dis- 

 tance of a lens or combination by holding it between a 

 light and a sheet of white paper or between a window 



and a sheet of paper 



and noticing how 



far distant it is 



from the paper when the image is clearest. It is well to 

 do this when purchasing these lenses, as many of them 

 focus at an inch and a quarter or more. Ten, divided by 

 the focal distance, gives the magnifying power in diame- 

 ters, ten is the distance in inches at which one usually 

 holds an object for close inspection, the magnifying power 

 of a lens of one inch focus is then ten diameters. 



Having obtained the lens the next step is to mount it 

 conveniently for use and this is one of the steps which 

 every student should take for himself. The one used by 

 the writer and which is here described, answers every 



