00 THE MICROSCOPE. 



instrument. The simple microscope may consist of one, 

 as seen in fig. 21, or of two or three lenses; if the 

 latter, then so arranged as to have the effect only of a 

 single lens. In the compound microscope, not less 

 than two lenses can be employed : one to form an in- 

 verted image of the object, which, being the nearest to- 

 the object, is called the object-glass ; the other to mag- 

 nify this image, and from being near the eye of the 

 observer, is called the eye-glass. 



I have so far considered a lens simply with reference 

 to its enlargement of the object, the increase of the 

 angle under which the object is seen. A further 

 and equally important consideration is that of the 

 number of rays or quantity of light by which every 

 point of the object is rendered visible ; and much may 

 be accomplished, as I have already pointed out, by the 

 combination of two or more lenses, which w r ill at once 

 reduce the angles of incidence and refraction. The 

 first satisfactory combination for the purpose was 

 the invention of the celebrated Dr. Wollaston. His 

 doublet (fig. 22) consists of two plano-convex lenses 

 having their focal lengths in the proportion of one to 

 three, or nearly so, and mounted at a distance which 

 is readily ascertained by experiment. The plane sides 

 of the lenses should be towards the object, and the 

 lens of shortest focal length next the object. 



It appears that Dr. Wollaston was led to this inven- 

 tion by considering that the achromatic Huyghenian 

 eye-piece, presently to be described, would, if reversed, 

 possess a power equal to that of the simple microscope. 

 But it will be evident, when the eye-piece is under- 

 stood, that the circumstances which render it achro- 

 matic are very imperfectly applicable to the simple 

 microscope, and that the doublet, without a nice 

 adjustment or a stop, would be valueless. 



The nature of the corrections which take place in 

 the doublet is explained in the annexed diagram, where 



1 o V is the object, p a portion of the cornea of the 

 eye, and d d the stop, or limiting aperture (fig. 22). 



Now it will be observed that each pencil of 

 light proceeding from I I' of the object is rendered 



