154 THE AMERICAN MONTHLY [July, 



A compound microscope on the other hand is always made up of 

 two or moi'e simple microscopes, so arranged that the magnified image 

 formed bv one simple microscope is viewed by another similar micro- 

 scope or else a second magnified image is formed with the second 

 simple microscope, and this in turn viewed by a third simple micro- 

 scope. When a compound microscope is made up of but two simple 

 microscopes, which is by far the more common kind, the object 

 viewed is not seen in its normal position but is reversed. In the case 

 of the vSpanish saflron stigma, the cleft end if at the right, would ap- 

 pear to be to the left. It is simplv turned around. A drop of a liquid 

 running down an incline would appear to crawl up hill in spite of the 

 force of gravity- Thus it will be readily seen that it is more difficult 

 to manipulate objects vuider a compound microscope than it is under a 

 simple instrument. By the use of an erector, which is another simple 

 microscope, the image can be again inverted so that it will appear in 

 its normal position. This is usually done when compound microscopes 

 are employed as dissecting instruments. 



Fortunatel}' for the pharmacist, the simple microscope is of great 

 service in his business, although I regret to state that it is not employed 

 to the extent it should be. There are several forms of the simple mi- 

 croscope on the market. Among the most serviceable are what an 

 Englishman has termed the platyscopic lenses. They come with three 

 separate lenses which magnif)' fifteen, twenty, and thirty (Hameters 

 respectively. Simpler-looking microscopes are made in this country 

 but do not magnify as much as these do. These English microscopes 

 are what are known as achromatic triplets. If these cannot be pro- 

 cured then purchase one of the cheaper simple microscopes, set in 

 rubber or tortoise shell (the rubber frame is the moi'e serviceable) and 

 make use of them. They are generally known as pocket magnifiers 

 and the lens closes into the frame like a knife blade into the handle. 

 If single ones are procured, get at least two, one with a power of fifteen 

 diameters and the other twenty or twenty-five diameters. I find a trip- 

 let which has three lenses in one case to be veiy convenient. 



It is by no means difficult to determine at least approximately the 

 magnifying power of a simple microscope. It is found in the case of 

 one lens by measuring the focal distance and dividing it by ten. The 

 focal distance is the distance of the lens from the object so that it can 

 be readily seen. Thus if the focal distance is one inch we divide ten 

 by one and get ten as the number of diameters the lens magnifies. If 

 the distance is but one-half inch then ten divided by one-half gives 

 twenty as the magnifying power expressed in diameters. Another 

 method which is based on the same principle is to focus some infinitely 

 distant object as the sun and then measure the distance of the lens from 

 the surface on which the sun is focussed and divide it into ten as in the 

 former case. When more than one lens is used their combined power 

 is equal to the sum of their individual powers. Thus a set of lenses 

 with powers of ten, fifteen, and twenty diameters respectively will 

 when combined equal forty-five diameters, and magnify the same as 

 a single lens of that power. In this connection it may be well to 

 state that whenever two or more lenses are used as a simple microscope 

 the higliest power or strongest lens should always be turned toward 

 the object. 



