ZOOLOGY AND BOTANY, MICEOSCOPY, ETC. 295 



of the compound Microscope is extraordinarily different. If we dis- 

 regard the faults of the image towards the margin of the field, and 

 consider only the maximum sharpness of the image in the region of 

 the axis, the eye-piece is, as a matter of fact, quite without influence. 

 In the simplest eye-pieces with unachromatic lenses, their action in the 

 centre of the field is practically free from error, when we estimate 

 the conditions under which they act. It is undoubtedly accurate, as 

 is often urged, that the eye-piece, even in the axis, exercises an influence 

 on the spherical and chromatic aberration of the pencils, but at the 

 same time as accurate as it is to say that the sun rises earlier for a 

 tall man than it does for a short one. 



For the proper performance of the Microscope, those faults in the 

 image are alone important which originate in the action of the 

 objective, and are only enlarged in the objective-image by the eye- 

 piece. From whatever cause these may spring, whether from external 

 imperfection of the lenses, or from aberrations, their common influence 

 consists in their imposing a certain limit to the useful magnifying 

 power in the case of every objective. The more perfectly an objective 

 of given focal length acts in both respects, the higher the magnifying 

 power it admits of by means of tube and eye-piece ; the more imper- 

 fect the union of the rays, the lower the magnifying power at which 

 the dispersion circles from each point of the image destroy its sharp- 

 ness and clearness. Moreover it is entirely unimportant in itself by 

 what means a given magnifying power is to be produced, whether by 

 longer tube and weaker eye-piece, or by shorter tube and stronger 

 eye-piece ; the amount of the united magnifying power is alone to be 

 considered, and must be compared to the magnifying power which 

 the objective, used as a magnifying glass, would give by itself. The 

 ratio in which tube and eye-piece may increase the available magnify- 

 ing power over that of the objective alone, without deterioration of 

 the image, forms the exact standard of the perfection of an objective. 

 On the one hand, this points out the reason why the attainment of a 

 higher magnifying power always necessitates objectives of shorter 

 focal length. This would not be the case if the objective could be 

 arranged so as to unite the rays perfectly, for nothing would then 

 hinder the production of any desired amplification of the image, 

 by means of tube-length and eye-piece, however great the focal 

 length of the objective might be. On the other hand, it is shown 

 that every advance in perfecting the objective, with regard to its 

 dioptrical functions, must enable amplifications, hitherto attained with 

 sufficient clearness by lenses of short focal length only, to be equally 

 well attained by lower power objectives. 



A comparison of the Microscope of the present day with those which 

 twenty, thirty, and forty years ago gave the best performance, shows 

 the steady progress which optics have made in this respect. Without 

 doubt it is of the greatest interest to examine what prospects there are 

 for the further perfecting of optical instruments in this direction. If 

 the opinion previously expressed on the importance of aperture and 

 on the extreme limit of microscopical perception is right, no im- 

 provement of the objectives in their dioptrical action can substan- 



