30 CONSTRUCTION OF 



press the stage C, as occasion requires. C consists of two plates of 

 brass held together by a spring ; between these plates the object slider 

 D is placed. E is an arm at right angles to A ; it is attached and con- 

 fined by the screw at the end of it in A. F must be supposed to 

 represent three lenses, or magnifiers, screwed together, the focus 

 of each being such that they act in unison together. G is a reflector 

 to cast the light upwards through the object to the eye. H is the main 

 tube, which bears the eye-lens at one end, and screws upon one of the 

 object lenses at the other. 



When the instrument is in use, the focus is adjusted by moving 

 the screw B, and the degree of magnifying power is according to the 

 lenses, which are screwed on to the bottom of H. Thus, if one lens 

 obtain an increase of 10, two lenses may obtain a power of 100, 

 and three lenses of 1000. Let it be always remarked, that the more 

 lenses the more obscure will be the image. 



18. MEDICAL ACHROMATIC MICROSCOPE. Another form of com- 

 pound microscope is represented in the following outline, (fig. 15.) It is 

 particularly adapted for the examination of anatomical and physiological 

 preparations, and from being less complicated in its arrangements 

 is also much less expensive. The stout stand 3 is screwed into the 

 foot 4, and the body 1 is supported by the arm 2. The object-glasses 

 are at 5, the eye-glass and field-glass are of course contained in the 

 body of the instrument ; 6 is the moveable stage, and 7 the mirror. 

 The portability, and at the same time high magnifying power of these 

 , instruments, renders them exceedingly useful. * 



Figure 16 is a more recent form of the same instrument ; but as 

 its chief difference consists in being arranged in such a way as to allow 

 of the instrument being brought into any convenient position by 

 the joint at 3, no more lengthened detail is requisite. Its internal con- 

 struction is essentially the same as the former. 



