410 



NATURE 



[March 2, 189; 



ance of small apertui-es. Low amplifications are as 

 useful in their own department as high ones ; and to put 

 great apertures to lower magnifying powers than such 

 magnifying power warrants is to sin against the elemen- 

 tary principles of the Abbe theory of vision. And on 

 the other hand, wide apertures can never be utilised unless 

 there is a concurrent and suitable linear amplification of 

 the image which is competent to exhibit to the eye the 

 smallest dimensions which are by optical law within the 

 reach of such apertures. 



Thus it follows that great amplification will be useless 

 with small apertures. If the power be deficient the 

 aperture will not avail ; if the aperture be wanting 

 nothing is gained by high power. The law is, " Employ 

 the full aperture suitable to the power used." In Abbe's 

 words, " A proper economy of aperture is of equal im- 

 portance with economy of power." ^ 



Taking these facts, then, which are apparently recog- 

 nised by Dr. Van Heurck, it is very remarkable to find on 

 page 49, in a discussion of the " screw threads" or gauges em- 

 ployed by the makers of microscopes,that the general value 

 of the English gauge is admitted, but it is added, " The 

 English thread is not, however, all that we have to say on 

 this matter. In America the Antericatt thread is also 

 employed, which is considerably greater, and admits the 

 use of lenses with a much larger diameter, and thus offers 

 certain advantages. In the first place, the larger the 

 lens the easier it is to make, and consequently the real 

 curvatures approach closer to the calculated curvatures ; 

 then the larger the lens the more luminous rays it admits, 

 and this in photography is not to be despised." 



To our judgment this statement is a contradiction of 

 the admission made on page 49, quoted above. 



The enlargement of the screw for the purpose of 

 putting in larger back lenses to objective combinations 

 was first mooted in America in 1879,^ when "Mr. 

 Bullock urged the desirability of adopting a uniform 

 objective screw of larger size than the Society screw now 

 in use (1879), ^.s being essential to the efficacy of low 

 power lenses of high angle" 



This " American gauge " was subsequently introduced 

 and known as the " Butterfield gauge of screw for 

 objectives." ^ 



Now, we must remember the date of the introduction 

 of this large gauge for objectives, and its relation to the 

 introduction of the apochromatic system of lenses. We 

 must further remember that the purpose of its adoption 

 was to permit the introduction of larger back lenses than 

 the Society gauge would suffer into an objective com- 

 bination. This meant giving relatively great apertures 

 to lower powers. But this, carried beyond a certain 

 limit, violated a fundamental law of Abbe's theory.* 

 Now it is said that these larger lenses are easier to 

 make (!) and approach more nearly to the calculated 

 curves. But in truth objectives with wide apertures 

 which are low powers, and must therefore have large 

 backs, are most difficult lenses to produce. It was, in 

 fact, to escape the difficulty of giving lower powers larger 

 angles that opticians of the first rank always designated 

 their objectives as of lower magnifying power than they 



1 J. R. M. S., ser. ii. vol. ii. p. 304. 

 '■^American Naturalist, vol. xiii. p. 60. 



3 J. R. M. S., ser. ii. vol i. p. 301. 



4 Ibid., vol. ii. p. 204. 



NO. 12l8. VOL. 47] 



really were. They in fact made a frds a ^ ; a i a troths ; 

 a Y*\)ths a 5 ; a j a i ; and so on. 



Since Butterfield's gauge was introduced, long before 

 the days of apochromatism, that is when our ignorance 

 allowed us to over-aperture our low magnifying powers, 

 it was tolerable, because it was evidence of experi- 

 mental effort to improve the capacity of our lenses. 

 But today with the society screw we are easily provided 

 with a series beginning with a i inch objective of -3, and 

 a \ inch objective of -65 N.A., and we may venture to 

 think that these are the highest ratios of aperture to 

 power that will be accomplished for many a day ; and 

 therefore the highest ratios allowable by the Abbe theory 

 of vision, which we now know, at least in this point, to 

 be an enunciation of the established laws of optics. 



Moreover, //^^j^ lenses are really difficult to make, with 

 their back lenses easily placed within the diameter cf the 

 Society screw. A high ratio of aperture to power always 

 involves great expense in production ; and therefore we 

 find that the low-priced oil immersions of this imme- 

 diate time are ,Vths and |ths, not objectives of low mag- 

 nifying power, and for this reason only. 



Since then the Society screw is sufficient for more 

 than double the apertures shown by Abbe to be in suit- 

 able ratio to the lower powers, we find it more than diffi- 

 cult to account for the teaching in a treatise intended to 

 be essentially elementary, that the Butterfield screw 

 gauge for objectives provides conditions which " offer 

 certain advantages/' when the supreme object of this 

 part of the book is to enunciate fully the nature and 

 qualities of oil immersion achromatic, and especially 

 apochromatic, object glasses, by which we can get larger 

 apertures wiih the society screw than in the old days of 

 Butterfield's gauge could be got by the use of abnormal 

 backs to objectives. We find also that '" penetrating 

 power " is referred to in passing as one of the properties 

 of object glasses (p. 56) ; but since the diffraction theory 

 of microscopic vision is associated with a special inter- 

 pretation of what this means, and since it is to Prof. 

 Abbe that we are indebted for placing this hitherto 

 obscure matter on a sound, scientific basis, it somewhat 

 disappoints the reader to find no allusion whatever to the 

 valuable work done on this subject, nor any elementary 

 endeavour to explain the great truth that the actual 

 depth of vision must always be the exact sum of the 

 accommodation depth of the eye and the focal depth of 

 the objective. But there are few matters of more prac- 

 tical importance or that lend themselves more to simple 

 exposition. 



In a treatise purporting to be essentially for the 

 beginner we confess to disappointment concerning the 

 instructions as to the " choice of a microscope." What 

 is needed is that the tyro should know the essentials of 

 the instrument ; the points in it that are of indispensable 

 importance, and a clear account of the manner in which 

 these may — by the uninitiated — be seen to be of inferior 

 or acceptable workmanship. The reader is not even in- 

 formed that in so important a matter as the fine 

 adjust)nent there is a different value to be attached to 

 several entirely different methods by which this function 

 of the microscope is performed. The bar and lever 

 movement, essentially the best in principle and practice, 

 is only referred to as existing, in the index, which is thus 



