30 v CONSTRUCTION OF THE MICROSCOPE. 



position ; but the points of the image formed by a lens emitting no 

 more than a small conical body of rays, which it receives from the 

 glass, can be visible only to the eye situate within its range. Thus 

 the pencil of rays emanating from the object at o is converged by the 

 lens iof, cross each other, and diverge towards h, and therefore would 

 never arrive at the lens fg, without the interposition of the plano- 

 convex lens at d e, placed at a smaller distance from the object ; and 

 by this means the pencil d n, which would have proceeded to h, is 

 refracted or bent towards the lens fg, having a radial point at p q. 

 The object is magnified upon two accounts : first, because if we view 

 the image with the naked eye, it would appear as much longer than 

 the object as the image is really longer than it, or as the distance fb 

 is greater than the distance from the real object to f; and secondly, 

 because this picture is again magnified by the eye-glass. The com- 

 pound microscope, then, consists of an object-lens, I n, by which the 

 image is formed, enlarged, and inverted ; an amplifying lens, d e, by 

 which the field of view is enlarged, and is consequently called i\\z field- 

 glass ; and an eye-glass or lens, by which the eye is permitted to ap- 

 proach very near, and consequently enabled to view the image under a 

 large angle of apparent magnitude. The two, when combined, are 

 called the eye-piece. 



Upon the construction of this microscope Mr. Ross observes : 

 " Since the power depends on the ratio between the anterior and pos- 

 terior foci of the object-glass, it is evident that by increasing that ratio 

 any power may be obtained, the same eye-glass being used or having 

 determined the first, any further power may be obtained by increasing 

 that of the eye-glass; and thus, by a pre-arrangement of the relative 

 proportions in which the magnifying power shall be divided between 

 the object-glass and the eye-glass, almost any given distance (within 

 certain limits) between the first and its object may be secured. This 

 is one valuable peculiarity of the compound instrument ; and another 

 is the large field, or large angle of view, which may be obtained, every 

 part of which will be nearly equally good ; whereas with the best sim- 

 ple microscopes the field is small, and is good only in the centre." 



Mr. Lister, as we stated in a previous chapter, first set about in- 

 vestigations which have ultimately proved of the utmost value. The 

 results arrived at by him were published in the Philosophical Transac- 

 tions ; and the principles have since been applied and exhibited by Mr. 

 Hugh Powell and Mr. Andrew Ross. It is due to the late Mr. Tulley 

 to say, that he constructed an achromatic object-glass of nine-tenths of 

 an inch focal length, composed of three lenses, transmitting a pencil of 

 eighteen degrees; and as regards accurate correction throughout the 



