March, 1913. 



KNOWLEDGE. 



103 



aperture) would often get the order, the tyro arguing that 

 increased N.A. meant more resolution and, therefore, better 

 results. It is altogether forgotten that behind everything is 

 the human eye, and that this admits of improvement within 

 but narrow limits." 



With the inference he draws from this I am entirely at 

 issue. The tyro does get more 

 resolution and better results out 

 of the larger aperture, within, 

 of course, certain well-defined 

 limits, though I doubt whether 

 A is foolish enough to give it for 

 nothing. I confess, indeed, to 

 finding it a little difficult to follow 

 Mr. Hutton in his arguments to 



prove the contrary. Certainly __^^^^fl 



there are limits to the ability of 

 the eye to separate lines beyond 

 a certain fineness apart. 1 can- 

 not, however, see the connection 

 of this with the microscope, 

 when the very object of the 

 instrument is to carry on definite 

 vision still further and further 

 than the unaided eye can 

 command. 



Up to a certain point I agree. 

 I admit, taking his words : " An 

 objective of 0-13 N.A. will 

 resolve twelve thousand five 

 hundred lines to the inch. . . . 

 To accomplish this the objective 

 must have an initial power of 

 ten, which, multiplied by ten 

 (the power of the eyepiece), 

 and then the one hundred 

 and twenty-five (the limit of resolution of ordinary eyesight), 

 gives us the twelve thousand five hundred." Granted, that if 

 such an objective were constructed, which with the ten-power 

 eyepiece would resolve just these 

 number of lines, and no more, i 

 granted also the limit mentioned 

 of the observer's vision, he would 

 not be able to pick up details 

 in the object of still more fine- 

 ness. Even if the objective 

 had double the N.A., could 

 resolve, in fact, lines of double 

 the fineness, and these in the ob- 

 ject; with the ten-power eye-piece 

 they would still remain invisible. 

 In this last case, however, what 

 is to prevent the tyro from 

 clapping on a twenty-power eye- 

 piece, when they would at once 

 be seen ? For myself, I can 

 separate two hundred and fifty 

 lines to the inch, yet it often 

 occurs that details invisible 

 under an eight-power eyepiece 

 become clearly defined under 

 an eighteen or even twenty- 

 seven. But, then, I do not con- 

 fine myself to objectives with 

 narrow apertures. To construct, 

 indeed, an objective with an 

 N.A. so limited as to show 

 things under a ten-power eye- 

 piece only ; then another of 

 double the N.A. to be able 

 to resolve structure of double 



the fineness, and no more, under the same conditions ; 

 would be, to my seeming, to emulate the man who cut 

 two holes in the door for his cats to go through, the 

 larger one for the mother cat, the smaller for her kitten, 

 oblivious of the fact that the larger aperture would suffice for 

 both. Here, as in other things the greater includes the less. 



\ 



Figure 96. A scale of Butterfly, Vanessa atalanta, 



magnified four hundred and seventy diameters. 



Taken by an objective of 0-17 N.A. 



Figure 97. The same scale taken by Swift & Son's 

 new two-thirds inch apochromatic objective of 0-30 

 N.A. Magnified four hundred and seventy diameters, 

 as before. 



microscopical readers of 



A micro-objective with reserve of aperture is a whole battery 

 of lenses in itself, the progress from lower to higher magnifica- 

 tion being made by changing the eyepieces instead of the 

 objectives. Five eyepieces ranging from four to twenty-seven 

 powers will give, with a two-thirds inch objective, on a seven- 

 inch tube, magnifications of from forty to two hundred and 



seventy diameters. 



I am assuming, of course, 

 that the lenses are good ; also 

 that deep eyepiecing refers 

 mostly to the low and medium 

 powers. With an oil immersion 

 one-twelfth inch I have never 

 been able to work with advan- 

 tage with anything higher than 

 a twelve-power eyepiece, calcu- 

 lated upon a ten-inch tube. 

 Here, again, I find myself at 

 issue with Mr. Hutton. He 

 says, when speaking of medium- 

 power lenses, ranging from the 

 one-inch to the half-inch or 

 four-tenths of an inch : — " These 

 admit abundance of light, and 

 often possess a working distance 

 equal to three-quarters of their 

 focal length. They can scarcely 

 be said to be used for highly 

 critical work, and will (not ? — 

 T. F. S.) bear an eyepiece mag- 

 nification at any rate more than 

 ten." Then: — "So that nowa- 

 days every objective of four- 

 tenths of an inch or lower will 

 bear an eyepiece magnifying ten 

 on the one hundred and sixty 

 millimetres tube. Even then we must be careful as to the 

 illumination, or we shall only get a foggy glare." 



To me this statement is truly astonishing, since, if there is 

 one thing more than another 

 which distinguishes these low 

 and medium powers from the 

 highest, it is their capacity to 

 stand deep eyepiecing. Why, 

 some of my lower powers only 

 begin to do their work when 

 under a twenty-seven eyepiece. 

 This especially applies to a new 

 two-thirds apocbromat of Swift 

 and Son, just acquired, of 0-30 

 N.A.; also to a half-inch of 

 theirs of 0-50 N.A., and a B.B. 

 of Zeiss of the same aperture, 

 all in my possession. I only 

 speak of my own glasses, but 

 others, no doubt, of other makers, 

 will do the same. Knowing what 

 my own lenses will do under 

 my own microscope, it would 

 almost seem that Mr. Hutton's 

 labours had ceased with compil- 

 ing his figures, and that he had 

 taken no further trouble to verify 

 his conclusions. I should be 

 sorry to misrepresent him, yet 

 what is one to think when 

 theory and fact are so far apart 

 as here ? 



Happily, the micro-camera is 

 ready to come to our aid to 

 judge between us, and let the 

 Knowledge" be the umpires. 

 True, in theory, photography is supposed to confer extra 

 resolving power upon the micro-objective, yet in practice I 

 have never found it so. In this opinion also, I am con- 

 firmed by Mr. Andrew Pringle, whose authority on the subject 

 is second to none. I remember many years ago attending a 



