OPTICAL PRINCIPLES OF THE MICROSCOPE. 15- 



the negative aberration occasioned by covering the object with a thin 

 plate of glass. This correction will obviously be more important 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 objectives of of an inch focus, whose angle of aperture does, 

 not exceed 75, may be made to perform very well without adjustment,, 

 if their corrections be originally made perfect for the average thickness 

 of glass used to cover objects of the finer kind. And objectives of much 

 higher power and larger angle of aperture (especially suited for Students'" 

 Microscopes), are now constructed so as to work admirably without 

 adjustment, being corrected for a standard thickness such as 0.008 or 

 0.006 inch of the glass covers supplied by their makers. Such non-adjust- 

 ing objectives, when less than inch focus, are best constructed on the 

 ' immersion ' system ( 19). 



18. For many years the best Microscopic objectives of moderate and 

 high magnifying power were made by combining three superposed pairs of 

 increasing focus and diameter (as in Fig. 10), each consisting of a double- 

 convex lens of crown-glass partly achromatized by its own concave of 

 flint; the two apposed surfaces of each pair being of the same curvature, 

 and cemented together by Canada balsam. Various modifications of this- 

 arrangement, however, have been introduced at various times and by 

 various constructors; some proceeding in the direction of simplification,, 

 whilst others have aimed at the greatest attainable perfection, irrespec- 

 tive of complexity and constructive difficulty. It is obvious that there 

 are great practical advantages on the side of any reduction in the number 

 of component lenses, that is compatible with the good performance of the 

 combination: liability to error, as well in the curved surfaces, as in the 

 centering and setting of each, being thereby diminished, while there is a 

 like diminution in the loss of light which occurs whenever the rays pass 

 out of one medium into another (1). But, on the other hand, it seems 

 certain that the highest theoretical perfection can be attained by an 

 increase in the number of component lenses; so that, if the errors in work- 

 manship are kept down to the lowest possible point, the performance of 

 such complex combinations may be made superior to that of simpler 

 ones. The first important change in the direction of simplification con- 

 sisted in the replacement of ihoj'ront combination by a single plano-con- 

 vex of crown. This substituti^, which seems to have been first devised 

 by Amici, has been very generally adopted; a greater working distance 

 from the object (which is very important in the case of the highest pow- 

 ers) being attainable in this construction, than when the front is either a 

 doublet or a triplet combination. But most makers who have used this 

 method have added a lens to the back combination, making it a ' tele- 

 scopic triplet,' still using a doublet in the middle; and admirable objec- 

 tives on this construction (each consisting of two flint concave and four 

 convex lenses of crown, with twelve surfaces in all) have been made by the 

 best Opticians English and American, French and German. A further 

 simplification has been recently carried into effect by Mr. Wenham; who 



