March 17, 1887] 



NA TURE 



467 



devoted to the consideration of speech, in which the 

 conditions of " word-deafness " and " word-blindness " are 

 now discussed. The view which has been put forward, 

 amongst others, by Ur. HughUngs Jackson, that there 

 are other and higher centres, over and above those which 

 have been demonstrated by physiological and clinical 

 research, which form the substrata of the higher metital 

 operations, does not receive support fro.n Ur. Ferrier. 

 " It seems more reasonable to believe that there may be 

 higher and lower degrees of complexity in the same 

 centres than to assume the seijarate existence of more 

 highly evolved centres, for which no evidence is obtained 

 by the results of experimental research " (p. 460). 



It might have been expected that the remarkable con- 

 ditions of the cerebral functions which are met with in 

 both man and animals in an hypnotic state would at least 

 have been alluded to in this edition, but they appear to 

 have entirely failed to attract the author's interest, and 

 the subject is passed over in silence. 



In conclusion it may confidently be affirmed that, 

 whatever exception may be taken to this or that state- 

 ment or opinion, or to the mode in which this or that 

 question is discussed, this new edition of Dr. Ferrier's 

 work, from the care with which it has been revised, the 

 extent of the information which it contains, and the clear- 

 ness of style and lack of ambiguity which characterise 

 its every page, must prove of the greatest \alue to the 

 student of neurology, and cannot fail to enhance the 

 high reputation of its author. E. A. Sch.^FER 



THE VALUE OF THE NEW APOCHROMATIC 



LENSES 

 A T the annual meeting of the Royal Microscopical 

 ■^*- Society, the Rev. Dr. Dallinger, who was elected 

 President for a fourth year, delivered his annual address, 

 in the course of which he gave a judgment concerning the 

 new object-glasses made with the new German glass, in 

 the following terms : — 



In proceeding to fulfil the honourable duty that, by 

 your courtesy, devolves upon me, I purpose in the 

 main to follow the line I have taken in preceding years. 

 I congratulate the Society on its work, and on its steady 

 influence in promoting progressive improvements in the 

 optical and mechanical construction of the microscope, 

 devoid of all prejudice as to how, or from whence, such 

 improvements may come. And whilst, happily, it is not 

 of necessity a President's duty to pass in cursory review 

 the microscopical work of the year, there are times when 

 it may be well for him to review the points of improve- 

 ment that have been made in the instrument itself. 



For the past twenty years I have had an increasing 

 interest in the co.itinuous improvement of the optical 

 appliances of our instrument— an interest which, from 

 the first, applied not only to objectives, but also to eye- 

 pieces and condensers, which consecutive calculation, 

 thought, and experience have shown to have a correlated 

 importance. 



Eighteen years ago I had, by practice, made myself 

 fairly master of a l'25inch objective of that period 

 made by Powell and Lealand. I still possess that lens, 

 and it is as good a lens of its class as they ever con- 

 structed. Soon after, 1 became equally familiar with a 

 I 50-inch of the same class by the same makers. 



By saying that I beca lie master of these lenses, I 

 mean that I discovered exhaustively what they would and 

 what they would not do. By this, I learned definitely 

 what 1 wanted in lenses, if I could get it ; and to get 

 that has been my unceasing endeavour until now. And 

 certainly the quest has not been vain. And my method 

 has been to examine impartially, and possess myself of, 

 English, Continental, or American lenses, whenever they 

 have shown any capacity for doing best what my work 

 proved to me required to be done. 



I know that, in estimating the quality of a lens by the 

 class of image it affords of certain test-objects well known 

 to us, a certain amount of empiricism must take place. 

 We do not absolutely know the image it ought to present. 

 But this only applies within very narrow limits. Take 

 the Podura scale : I can give you an image of it with 

 my I '25-inch and i 50-inch of twenty years ago. What 

 I, in common with most micros:op'.sts, considered then 

 the best result, the most sharp, clear, and delicately 

 defined image, with those lenses I can get now ; but, with 

 those lenses, nothing better. 



But the elements — the essential features that consti- 

 tuted the quality of beauty in that image — are the very 

 elements, the actual features, that every admitted im- 

 provement in our object-glassas has brought out more 

 perfectly. So that if I now put, say, the Podura scale 

 under my old dry i/25-inch objective, and, beside it, 

 another precisely similar scale under a new homogeneous 

 I 20-inch objective of N.A. (numerical aperture) i'5, the 

 very qualities of the image which I, and experienced 

 microscopists generally, thought the best twenty years 

 ago are incomparably transcended in beauty and perfect- 

 ness now. 



But that is not, and has not been, my only or my chief 

 test. It has been one more eminently practical, so far 

 as my own work went ; at least for some years. 



Up to ten years ago, although I had spent weeks in 

 patient effort, no lens that I possessed, or that was within 

 my reach, could be made to reveal the flagella of Bacte- 

 rium termo. The flagella of many minute monads and 

 of such Bacterial forms as Spirit/ian volutans, and even 

 Bacterium lineola, I could demonstrate, though some of 

 them with difficulty ; but not a trace of that of B. termo. 

 But, near that time, Powell and Lealand produced a bat- 

 tery of immersion-lenses on a new formula and of much 

 relative excellence ; and with these lenses the flagella of 

 B. termo were brought within the range of sight. 



Since that time that has been a good lens, to me, in 

 proportion to the greater or less ease and perfection with 

 which it has revealed this delicate fibre. And let me say 

 that such lenses as do this are those that always, without 

 fail, give us the best ideal image of Podura scales and 

 other tests. Vou will pardon me, I trust, for this amount 

 of personal reference, since it will give a greater relevancy 

 to what will follow. 



Improvements of great optical importance have been 

 made during the last few ye.irs. The manufacture of 

 homogeneous lenses by .Messrs. Powell and Lealand gave 

 us the opportunity, which we could not have with foreign 

 makers, of urging certain modifications. The addition 

 of the correction collar was a minor, but still important, 

 point. But the great point was the increase of the N.A. 

 These makers have shown themselves most anxious, and 

 have spared no efforts, to reach the highest aperture yet 

 attained. 



Advancing, say, from N.A, i '25, they attained to i'35 

 in such powers as the i, '25-inch and the i/ 50-inch ; sub- 

 sequently to r47 in i/S- and i/12-inch objectives; and 

 finding these, from my working point of view, of such 

 supreme gain, I urged them still on, and was ultimately 

 rewarded by the possession of a i 6-inch N.A. r5., fol- 

 lowed by a 112- and a i 20-inch foci of the same great 

 aperture. From each of these I obtained special advant- 

 ages over all like powers, but with lower apertures, within 

 111)- reach. 



A question frequently asked may be asked again, In 

 what way do these last increments of aperture aid us ? The 

 practical answer is not difficult. Speaking from observa- 

 tion, I may say th.it all the objectives I have employed 

 for the most critical work fail to produce images by the 

 extreme marginal zone of the aperture. It is the judg- 

 ment of competent judges that it will be fair to roughly 

 estimate this defective outermost zone at 10 per cent. ; so 

 that, from the total measurement of the aperture by Prof. 



