MICROSCOPES. 331 



Compound lenses of short focal length, properly corrected for 

 viewing an uncovered object, will not be properly corrected 

 when the object is covered with a piece of thin glass. To make 

 the requisite correction such obj -ct-glasses are usually provided 

 with a screw-collar, by means of which the front compound lens 

 can be brought nearer to the other, so as to compensate for the 

 effect of the light passing through a greater or less thickness of 

 covering glass. By this means correction may also be made for 

 the effect of increasing the distance between the object-glass and 

 the eye-piece, by drawing out the so-called draw tube. This 

 method of increasing the magnifying power is sometimes very 

 useful, but should not be pushed beyond moderate limits, since, 

 strictly speaking, the object-glasses must be corrected for one par- 

 ticular distance between them and the eye-piece. This distance 

 is often made considerably shorter in continental than in English 

 microscopes. 



Some object-glasses are made of several independent portions, 

 which can be used alone or combined. The only advantage in 

 such an arrangement is the diminished cost of the instrument ; 

 but this advantage is obtained by sacrificing quality, since perfect 

 correction depends on the combined action of all the lenses, and on 

 removing one or more, the rest cannot be perfectly correct. 

 When, however, cost is a very important consideration, this prin- 

 ciple may be made use of with perhaps better results than could 

 be obtained at the like cost by any other means. 



EYE-PIECE. 



The eye-pieces of compound microscopes consist of two plano- 

 convex lenses, called respectively the field-glass and the eye-glass 

 the one, further from the eye, being of very great use in 

 increasing the size of the field of vision, and the one nearer to the 

 eye being instrumental in still further magnifying the enlarged 

 inverted image. Both combined play a very important part in 



