856 SUMMARY OF CURRENT RESEARCHES RELATING TO 



Consequently, in the case assumed above of a power (N) of 800, x is 

 rather more than 0*0009 mm. (or 0-9 /*), or if the object is in a 

 medium of n = 1 ■ 50, not quite 1 ■ 5 /*. This exceedingly slight 

 displacement of the focus forms the measure of the alteration 

 in the path of the rays in the objective, and it is the aberra- 

 tion which corresponds to the difference in the visual distance, 

 assuming that accurate correction is first made for x = 0: much 

 less if the largest possible aperture is assumed for that x, and 

 generally not ascertainable, since it depends, like the moving 

 of the lenses towards each other (by the correction-collar) upon 

 the particular construction of the objective. Let us, however, 

 assume that this movement of the lenses which is necessary for the 

 equalization of the very slight difference in the path of the rays 

 corresponding with the above ascertained difference of adjustment 

 (and of the consequent disturbance of the spherical correction), 

 amounts to even 0*1 M , or 0-0001 mm., which is certainly far too 

 high, this would still be a quantity which is unattainable by any 

 mechanical contrivance, least of all by such a mechanism as the cor- 

 rection-collar. If Dr. J. Edwards Smith adduces against the results 

 thus established by theory, a case in which three divisions on the 

 scale of the correction-screw would be required for the equalization 

 of the difference between the power of accommodation of his own eye 

 and that of another observer (Mr. C. Spencer), it must be said that 

 such a thing is entirely absurd. It proves in fact simply that what 

 he regarded as the action of different powers of accommodation, was 

 nothing more than an effect of " personal equation " in the judgment 

 of the best image, and therefore rests entirely on purely subjective 

 opinions. 



If we now further examine the matter from a practical point of 

 view, it may be at once allowed that the technical considerations 

 against the correction-collar are not so weighty that it should be set 

 aside on that account if really practical advantages were to be gained 

 by it. For even if the greatest perfection of centering (such as is 

 possible with the fixed mounting) cannot be obtained with the cor- 

 rection-collar nor its durability guaranteed, yet according to the 

 examination by Professor Abbe of the correction-objectives of Powell 

 and Lealand and Zeiss, a sufficient amount of accuracy can be obtained 

 by very careful work. The question of expense, which the author 

 previously laid stress on, need not be considered, because, as Professor 

 Abbe observes, the technical difficulties in mounting the fixed ob- 

 jectives (on account of the final adjustment of the distances of the 

 lenses to fractions of a hundredth part of a millimetre), are not less 

 than in those with the correction-collar, and therefore the price for 

 both kinds is about equal. In this respect, therefore, there is nothing 

 to urge against the introduction of the correction-collar. 



Now, however, the question arises, how and to what extent the 

 possible advantages suggested by theory can be realized in practice 

 without prejudicing the usefulness of the objectives, and on this point 

 the author is strongly convinced that in the proper scientific use of the 

 Microscope for the examination of unknown objects and structural 



