THE COMPOUND MICROSCOPE. ' 67 



CHAPTER II. 



COMPOUND MICROSCOPE. 



A COMPOUND microscope differs from a simple cue in havino- 

 the image of an object formed by an object-glass further 

 magnified by one or more lenses forming an eye-glass ; or, in 

 other words, the rays of light from an object being brought 

 into a new focus, there form an image, which image being 

 treated as an original object by the eye-piece, is magnified in 

 the same way as the simple microscope magnified the object 

 itself. For a microscope to be a compound one, it is, there- 

 fore, necessary that it should have an object-glass and an eye- 

 glass; in some of the old microscopes there were only two 

 lenses, but it has been stated that, in the simple microscope, 

 as many as three are employed to form a triplet, and yet, 

 with this number of lenses, the microscope is still a simple 

 one. This is easily explained: the first two lenses of the 

 triplet only effect what might have been accomplished, but 

 not so well, by one ; and the third lens is only useful for 

 modifying the light before it enters the eye. 



As the object of this work is entirely practical, no mention 

 will be made of the compound microscopes that have been 

 heretofore, or are even now, manufactured in this country, 

 that are not achromatic, and which, therefore, are unfitted 

 for scientific investigation, and attention will be principally 

 directed to those made by our first-rate opticians, Messrs. 

 Powell, lloss, and Smith, all of whose object-glasses will 

 stand the severe tests hereafter to be described, approaching, 

 as far as we can judge at present, the limits of conceivable 

 perfection, and the stands or supports for which ar& con- 

 structed on the most approved mechanical principles, to 

 prevent tremor, and to afford the greatest facility for using 

 the various movements, and, in point of workmanship, are 

 also unequalled. 



Every compound microscope may be said to consist, like 

 the simple one, of two essential parts, viz., the stand and the 

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