OF THE MICROSCOPE. 165 



corrected his bad methods, but then he evidently had never 

 considered it worthy of earnest thought, although it formed 

 the very foundation of his powers of observation. 



Simple Hand Magnifiers. These are perhaps the most 

 important of all optical instruments, and yet we rarely find a 

 person who can use them efficiently. There are but three 

 points that require attention, viz : The proper position of the 

 magnifier itself, the perfection of the illumination, and the 

 steadiness with which the instrument is held at the exact focal 

 distance from the object. Many magnifiers are so constructed 

 that it is impossible to place them in a wrong position; the 

 side which should go next the eye, and the side which should 

 go next the object are so well marked that no mistake can be 

 made. The greatest liability to error exists where two or three 

 lenses of different powers are fixed in the same frame and used 

 together. This forms one of the most common and useful of 

 our magnifiers, and the rule is always to place the lens of great- 

 est power nearest to the object. Plano-convex lenses should be 

 placed with the plane or flat side next the object. 



Hand magnifiers are, in the majority of cases, used for ex- 

 amining opaque objects, and one of the most important con- 

 ditions for perfect vision is that the object be well illuminated. 

 First of all, then, see that the light falls full and direct on the 

 object; then place the magnifier as nearly in focus as can be 

 done without actually looking through the lens, and, after this, 

 approach the eye to the magnifier. The errors most commonly 

 committed are: Turning the object away from the light; cut- 

 ting off the light by the projecting brim of a hat or cap; 

 shading the object by the hand or the lens itself; attempting to 

 examine an object in a room that is not sufficiently lighted. 



Having secured a proper position for the magnifier and a 

 good illumination, the next step is to devise some means for 

 holdiog the lens steadily in focus during the examination. This 

 is most readily effected by resting the hand that holds the lens 

 upon the hand that holds the object. Lens and object then 

 move together, and the focussing remains unchanged. 



Compound Microscopes. We presume that the instru- 

 ment in hand is a very simple one, and that tl^ magnifying 



