\YITH THE MICROSCOPE. 



of the microscope above the object-glass, and, like the same arrange- 

 ment in the telescope, reverses the image. 



The magnifying power of the compound microscope may be 

 augmented either by increasing the power of the object-glass or that of 

 the eye-piece, or by increasing the distance between the object-glass 

 and the eye-piece. It must be borne in mind, however, that in 

 increasing the power of the eye-piece we do not magnify the 

 object itself in a greater degree, but simply increase the size of 

 the image of the object formed by the object-glass. Any imper- 

 fections which may exist in the object-glass are thus greatly 

 augmented. Hence we should never work with deep eye-pieces, 

 but when we wish to magnify an object more, we should adapt 

 a higher power to the instrument. Information upon employing very 

 high powers will be found near the end of the work. It will be 

 convenient for me to allude in the first place to the optical portion 

 of the microscope, and secondly to the mechanical appliances for 

 moving the object, altering the focus, &c. The optical portion in- 

 cludes the eye-piece, object-glass, and the mirror from which the 

 light is reflected so as to pass through the object. 



Optical Portion of the Microscope. 



4. Negative Eye-piece.- The eye-piece in ordinary use is the nega- 

 tive or Hitghcnian eye-piece, fig. 3, pi. I. It consists of two plano- 

 convex glasses, the flat surfaces of each being directed upwards. 

 The one nearest the eye of the observer is the eye-glass, and the one 

 at the greater distance the field-glass. 



Reiner's eye-piece is made like the above, but the eye-glass is an 

 achromatic combination. At the suggestion of Mr. Brooke I have 

 lately used this eye-piece as a condenser with the best results. 



5. The Positive Eye-piece, of Ramsden, is only used in those 

 cases in which it is necessary to see distinctly some object in 

 the eye-piece, as an instrument for measuring, at the same time that 

 the object itself is in focus. In this the convex surfaces of each of 

 the two glasses are directed towards each other as represented in 

 fig. 4. 



G. Object-glasses. The object-glasses, fig. 7, pi. I. used in the best 

 instruments are of English manufacture, but some of those furnished 

 with the cheap microscopes are made on the Continent, and are 

 much less expensive. 



The two most useful object-glasses for the student are the quarter 

 of an inch which, with the No. i eye-piece, should magnify from 200 

 to 220 diameters, and the inch which should magnify from 30 to 40 



