COKRECTION FOE COVERING OF THE OBJECT. 77 



its positive aberration is more strongly exerted upon the other 

 pairs, than it is when the distance between the lenses is increased, 

 and the distance between the front pair and the object is dimi- 

 nished. Consequently, if the lenses be so adjusted that their cor- 

 rection is perfect for an uncovered object, the front pair being 

 removed to a certain distance from the others, its approximation 

 to them will give to the whole combination an excess of positive 

 aberration, which will neutralize the negative aberration occa- 

 sioned by covering the object with a thin plate of glass.' It is 

 obvious that this correction will be more important to the perfect 

 performance of the combination, the larger is its angle of aper- 

 ture ; 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 there- 

 fore will be the negative aberration produced, which will, if un- 

 corrected, seriously impair the distinctness of the image. And 

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

 aperture is comparatively small ; nor even for the higher, so long 

 as their angle of aperture does not exceed 50°. As a large pro- 

 portion of the lenses made by foreign Opticians do not range 

 beyond this, the adjustment in question may be dispensed with ; 

 and even where the angle is much larger, if the corrections be 

 made perfect for a thickness of glass of 1-lOOth of an inch (which 

 is about an average of that with which objects of the finer kind 

 are usually covered), they will not be much deranged by a dif- 

 ference of a few hundredths 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 Ifature. Their essential 

 diiference 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 subject only to a change in their 

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

 formed by a lens, which image is vi.ewed by the observer through 

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

 microscope may consist of orie lens ; but (as will be presently 

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

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

 corresponding to that of a single lens. In the comfound 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 ohject-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 



' The mode in which this adjustment is effected, will be more fitly described here- 

 after (§ 82). 



