36 VISION WITH THE COMPOUND MICROSCOPE 



CHAPTER II 



THE PRINCIPLES AND THEORY OF VISION WITH THE 

 COMPOUND MICROSCOPE- 



WE are now prepared to enter upon the application of the optical 

 principles which have been explained and illustrated in the foregoing 

 pages to the construction of microscopes. These are distinguished 

 as simple and compound, each kind having its peculiar advantages 

 to the student of nature. Their essential difference consists in this, 

 that in the former, the rays of light which enter the eye of the 

 observer proceed directly from the object itself, after having boon 

 subjected only to a change in their course, as we have shown by 

 fig. 26, which fully explains the action of the simple lens ; whilst in 

 the" compound microscope an enlarged image of the object is formed 

 by one lens, which image is magnified to the observer by another, 

 as if he were viewing the object itself. In the compound micro- 

 scope not less than two lenses must be employed : one to form the 

 enlarged image of the object, immediately over which it is placed, 

 and hence called the object-glass ; whilst the other again magnifies 

 that image, and, being interposed between it and the eye of the 

 observer, is called the eye-glass. A perfect object-glass, as we have 

 seen, must consist of a combination of lenses, and the eye-glass is 

 best combined with another lens interposed between itself and the 

 object-glass, the two together forming what is termed an eye-piece. 

 The compound microscope must be the subject of careful and de- 

 tailed consideration ; but it must be remembered that the shorter 

 the focus of the simple magnifying lens, the smaller must be the 

 diameter of the sphere of which it forms part ; and, unless its 

 aperture be proportionately reduced, the distinctness of the image 

 will be destroyed by the spherical and chromatic aberrations neces- 

 sarily resulting from its high curvature. Yet notwithstanding the 

 loss of light and other drawbacks attendant on the use of single 

 lenses of high power, they proved of , great value to the older micro-, 

 scopists (among whom Leeuwenhoek should be specially named), on 

 account of their freedom from the errors to which the compound 

 microscope of the old construction w r as necessarily subject ; and the 

 amount of excellent work done by means of them surprises every one 

 who studies the history of microscopic inquiry. An important im- 

 provement on the single lens was introduced by Dr. Wollaston, who 

 devised the doublet, still known by his name, which consists of two 

 plano-convex lenses, whose focal lengths are in the proportion of one 

 to three or nearly so, having their convex sides directed towards 



