20 SELECTION AND USE 



yet contain very few parts. Thus the little vertical French 

 microscopes, which cost only $2,50, are compound, although 

 very simple in construction, while a simple microscope, if bin- 

 ocular, and provided with all desirable adjustments, might be 

 a very complicated affair. The difference between simple and 

 compound microscopes is this: in the simple microscope we 

 look at the object directly, while in the compound microscope 

 we look at a magnified image of the object. In the simple 

 microscope, objects are always seen in their natural position, 

 while in the compound microscope they are inverted, and right 

 becomes left, and left becomes right. This makes it very diffi- 

 cult for beginners to work upon objects under the compound 

 microscope; and hence simple microscopes are almost always 

 used for dissecting and botanizing. 



It is true that by adding more lenses, and making the instru- 

 ment still more compound, we can again invert the image, and 

 thus bring it back to its original and natural position, and 

 almost all the very expensive microscopes are furnished with 

 these extra lenses arranged in a piece of accessory apparatus 

 technically known as an erector. The distinguishing feature of 

 the compound microscope remains, however, the same. Certain 

 forms of the microscope, in which concave lenses are substi- 

 tuted for the usual convex form, also give erect images, but 

 this does not affect the general truth of the statement just 

 made. 



Simple microscopes frequently consist of more than one 

 lens. Thus, in using the ordinary pocket magnifiers with two 

 or three lenses, it is usual to employ all the lenses at once, look- 

 ing at the object through two or three lenses at the same time 

 when a high power is required. In this case, however, the two 

 or three lenses are placed close together and act in the same 

 way as a single lens, with surfaces more sharply curved than 

 those of any of the lenses forming the combination. Under 

 such circumstances the image is not inverted, but if we now 

 separate the lenses sufficiently, we will find that on again bring- 

 ing the object into focus, the image is inverted and greatly 

 enlarged. Moreover, it will be found that the magnifying 

 power may be greatly increased by increasing the distance be- 

 tween the two lenses, and it will also be found that as the 



