THE MICROSCOPE. 



is a tabulated statement of the powers resulting from these com- 

 binations, the four eye-glasses being designated in the order of 

 their powers, by the letters A, B, c, and D. The prices of the 

 object-glasses severally are given in the last column of the table, 

 and the slightest reference to them will explain the general desire 

 of microscopists to diminish expense, by varying their powers, by 

 the expedient of separating the lenses which enter into the com- 

 position of the object-pieces. 



Achromatic Object-glasses for Microscopes. 



When angular apertures, so extreme as those indicated in the 

 preceding table, are attempted, it is necessary that the object- 

 lens presented to the pencil diverging from the object, shall be 

 of the meniscus form, the concave side being turned towards the 

 object, for the reasons explained in 19. 



Besides the larger class of instruments above described, Mr. 

 Ross constructs microscopes in a variety of other forms, which 

 are placed within the reach of those who do not find it conve- 

 nient to incur the expense of the larger instrument. 



MESSRS. SMITH AXD BECK'S MICROSCOPE. 



84. The largest and most efficient class of instruments con- 

 structed by these artists, do not differ much in their mounting 

 from those of Mr. Ross above described. Like the latter, they 

 are supported by a horizontal axis, between two strong vertical 

 pillars, screwed into a tripod base. The instrument with its 

 appendages, turning on the horizontal axis, can thus be placed at 

 any obliquity whatever with the vertical. The coarse adjust- 

 ment of this microscope is made by a rack and pinion, by which 

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