18 OPTICAL PRINCIPLES OF THE MICROSCOPE. 



to the perfect performance of the combination, the larger is its 

 angle of aperture ; since the wider the divergence of the oblique 

 rays from the axial ray, the greater will be the refraction which 

 they will sustain in passing through a plate of glass, and the 

 greater therefore will be the negative aberration produced, which, 

 if uncorrected, will seriously impair the distinctness of the image. 

 It is consequently not required for low powers, whose angle of 

 aperture is comparatively small, nor for medium powers, so long 

 as their angle of aperture does not exceed 50° ; and even objec- 

 tives of l-4th of an inch focus, whose angle of aperture does not 

 exceed 70°, may be made to perform very well without adjustment 

 if their corrections be originally made perfect for a thickness of 

 glass of 1 -100th of an inch (which is about an average of that 

 with which objects of the finer kind are usually covered), being 

 not much deranged by a difference of a few lOOOths of an inch, 

 more or less, in that amount. 



16. 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 been subjected only to a change in their 

 eourse ; whilst in the latter, an enlarged image of the object is 

 formed by a Lens, which image is viewed by the observer through 

 a simple microscope, as if it were the object itself. The Simple 

 Microscope may consist of one Lens ; but (as will be presently 

 shown) it may be formed of two, or even three ; these, however, 

 being so disposed as to produce an action upon the rays of light 

 corresponding to that of a single lens. In the Compound Micro- 

 scope, on the other hand, not less than two Lenses must be em- 

 ployed: one to form the enlarged image of the Object, and this, 

 being nearest to it, is called the Object-glass ; whilst the other 

 again magnifies that image, being interposed between it and the 

 Eye of the observer, and is hence 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 inter- 

 posed between itself and the object-glass, the two together 

 forming what is termed an Eye-piece (§ 21). — These two kinds of 

 instrument need to be separately considered in detail. 



2. Simple Microscope. 



1 7. In order to gain a clear notion of the mode in which a Single 

 Lens serves to ' magnify ' minute objects, it is necessary to revert 

 to the phenomena of ordinary Vision. An Eye free from any 

 defect has a considerable power of adjusting itself, in such a 



