104 



KNOWLEDGE. 



March, 1913. 



lecture on Photo- micrography given 

 graphic Society, now the Royal. Aft 

 asked a question as to the in- 

 creased resolving power photo- 

 graphy gave, instancing examples 

 in the sister science, where 

 more stars were seen upon the 

 negative than in the telescope. 

 A facetious gentleman among 

 the audience called out : " That 

 depends upon the plates," mean- 

 ing, of course, the more holes in 

 the plates the more stars. Mr. 

 Pringle's sober answer was that 

 although he had often seen 

 details of structure upon the 

 negative not noticed in the 

 microscope, when he referred 

 back to the microscope they 

 were always there visually. If I 

 may refer to a matter touching 

 myself personally, I may say 

 that though, according to the 

 Psalmist past the age of man, my 

 eyesight is as good as ever it 

 was. What the plate can see, I 

 can see in the microscope, and 

 that is the way I test it. 



The photo-micrographs here 

 given, then, may be taken as 

 true representatives of what 

 was seen, and not seen, in the 

 microscope. Figures 96 and 97 

 are from a butterfly's scale, 

 Figure 96 being taken with a 

 lens of 0-17 N.A., the outside 

 limit Mr. Hutton will allow for a 

 two-thirds inch. Figure 97 by 

 my own two-thirds apochromat 

 of Swift, N. A. 0-30, both mag- 

 nified four hundred and seventy 

 diameters, or forty-seven times 

 the initial power of the objectives 

 on a seven-inch tube. Now, can 

 anyone hesitate for a moment 

 in deciding which picture shows 

 the most detail ? Photographi- 

 cally, in the quality of the prints 

 there is not so much difference. 

 When we come to examine them, 

 however, we find that while 

 Figure 96 shows only the ribbing 

 running from end to end of 

 the scale, with just indications 

 of the coarser cross ribbing at 

 the tip ; in Figure 97 the cross 

 striations are crowded from end 

 to end. Neither can it be said 

 of the second that the magnifi- 

 cation (fiqual to a forty-seven 

 power eyepiece) is too much for 

 clear definition in the print, 

 nor again that the image has 

 broken down, or is foggy. I 

 would not for a moment, how- 

 ever, let it be thought that the 

 objective, by which the first 

 print was taken, is a bad one. 

 Indeed, it is very good, but, of 

 course, cannot show structure 

 beyond the limits of its aperture. 

 One might as well expect a pony 

 to do the same work as a dray 

 horse. 



So much for the lower and 

 medium powers. I now beg to 



by him before the Photo- 

 er the lecture a gentleman 



Figure 98. Part of a scale of the same Butterfly, 

 magnified one thousand five hundred diameters. 

 Taken by a half-inch of Swift & Son's, of 0-50 N.A., 

 abnormally magnified to compare in size with Figure 99. 



Figure 99. The same part of the same scale, 

 magnified one thousand five hundred diameters to 

 show bosses on the cross ribs. Taken by a one- 

 sixth inch of Swift tk Son's, of 0-85 N.A. 



deal with another statement concerning some of the higher, 

 and am sorry to say that to deal will be also to disagree. 



Mr. Hutton's ideal aperture for 

 a four millimetre (one-sixth inch) 

 is 0-52 N.A., and he says: "It 

 is, however, useless to give us a 

 four millimetre (one-sixth) ob- 

 jective of 0-88 N.A., as its 

 aperture could not be fully 

 utilised except by employing such 

 high power eyepieces, or length- 

 ening the tube, as would utterly 

 break down the critical character 

 of the image. Anything above 

 0-52 N.A. for such an objective 

 is of little or no value, and if 

 working distance is sacrificed to 

 obtain a higher aperture it is 

 worse than useless." 



Here, again, we can bring 

 photography in as an impartial, 

 though striking, witness in the 

 case. I am lucky also in being 

 able to reproduce the image of 

 the same object by the two aper- 

 tures, or nearly so, he mentions. 

 Figure 98 was taken by a half- 

 inch of 0-50 N.A.,and Figure 99 

 by a sixth-inch of 0-85 N.A., 

 both of Swift. The first falls 

 two points below Mr. Hutton's 

 ideal, it is true, the other, three 

 points below the one he repro- 

 bates, yet I doubt whether it will 

 affect the definite picture in either 

 case. Both are taken at twenty- 

 five times the initial power of a 

 one-sixth, being, so far as the 

 half-inch is concerned, equal to 

 the initial power of seventy-five 

 times upon a ten-inch tube. My 

 sole object in this last was that 

 the two objects of the same 

 size might be compared, fifteen 

 hundred being far past the 

 point, on this glass, of useful 

 magnification. 



Figure 98, however, teaches a 

 useful lesson in another way. 

 Practically, there is not much 

 more seen of the structure than 

 is shown in Figure 97, simply 

 long girder ribs with short ones 

 at right angles between. In 

 Figure 99 one sees that the cross 

 ones are beaded, pointing to 

 further structure in the scale. 

 This beaded appearance is due 

 to little bosses, which, rising 

 from the cross ribs, support a 

 structureless membrane on the 

 top. Now, the half-inch being 

 of 0-50 N.A. gives a slight hint 

 of this structure, though not 

 enough to be of much service, as 

 it appears under a low eyepiece. 

 So far, however, from a deep 

 eyepiece helping, it only obliter- 

 ates entirely what was but 

 slightly indicated before. The 

 reason is not far to seek. Assum- 

 ing that a twenty power eyepiece 

 is substituted for the one of 

 ten, each point of structure has 

 only one quarter of the light. 

 This being now insufficient 



