THE MICROSCOPE 33 



THE COMPOUND MICROSCOPE 



The compound microscope consists of a series of 

 plano-convex or converging lenses. These are mutu- 

 ally arranged so as to form two systems of lenses an 

 objective or lower combination and an ocular or upper 

 combination. The objective, or set of lenses nearest 

 the object, produces an enlarged image which is again 

 magnified by the ocular lenses. Therefore the image 

 produced by a compound microscope is the result of 

 a double magnification. The object is placed beyond 

 the focal point of the objective and the picture pro- 

 jected by this system of lenses is an inverted real 

 image. This image is formed at a point within the 

 focal distance of the ocular; therefore the picture 

 formed by the ocular will be a virtual magnification 

 of the real image produced by the objective. The 

 object as viewed through the compound microscope 

 always appears inverted (Fig. 18). The field or area 

 which can be viewed is always smaller than that covered 

 by the lens of a simple microscope. The magnifica- 

 tions obtainable range from 15 to 2500 diameters, 

 although little practical application is found for mag- 

 nifications above 1500 diameters. The principal parts 

 of a compound microscope are the objective and the 

 ocular; but mechanical devices are necessary to hold 

 these lens systems hi proper relation to each other 

 and to the specimen. Proper support and illumi- 

 nation of the object must also be provided for. It is 

 therefore customary, in considering the construction 

 of a compound microscope, to speak of the optical 

 parts and the mechanical parts. 



