OF THE MICROSCOPE. 19 



While the magnifying power of lenses depends upon their 

 focal length, this in turn depends upon the material of which 

 the lens is made, and also upon the curvature given to its sur- 

 faces. Lenses of precisely the same form, and made respect- 

 ively of diamond, flint glass, crown glass and Canada balsam 

 would possess different magnifying powers; the diamond mag- 

 nifying most, the flint glass next, crown glass next, and Canada 

 balsam least of all. On the other hand, of two lenses composed 

 of the same material, that which has the sharpest curvature to 

 its surfaces will magnify most. Now, on reflection, it will be 

 evident to even the least mathematical mind that lenses which 

 have very sharp or quick curves must of necessity be small. 

 Suppose the curve which bounds the figure of a lens has a 

 radius of half an inch, it is evident that the largest lens which 

 could be made with this curve woiild be one inch in diameter, 

 and then it would be a perfect sphere. Most lenses, however, 

 resemble thin slices off the spheres, or in some cases two such 

 slices joined together, so that the diameter of the lens is in 

 general greatly less than the radius of the curves which form 

 its surface. Therefore, we see that all lenses of high power are 

 of necessity small, and when lenses are required of very high 

 power they become so minute as to be handled only with great 

 difficulty. Indeed, before the modern improvements in the 

 microscope, many of the lenses used by scientific men were 

 nothing more than little globules of glass, brought to a round 

 form by fusion. 



We have made this lengthened explanation of a very simple 

 matter because we have found amongst beginners in micro- 

 scopy a very general idea that large lenses are the most power- 

 ful. " Send me one of your largest and most powerful mag- 

 nifiers," is an order with which every optician is familiar, and 

 yet such an order contains a contradiction in terms. A lens 

 cannot possibly be large and magnify greatly at the same time. 



The Different Kinds of Microscopes. Microscopes 

 are divided into two classes simple and compound the dif- 

 ference between them being purely optical, and not mechanical; 

 for a simple microscope may be very complex and expensive, 

 while; on the other hand, a microscope may be compound and 



