Septi;mi!i:r, 191:i 



KXdWLI'.DGE. 



347 



PHOTOGRAPHY. 



By Edgar Senior. 



PHOTO-MICROGRAPHY WITH MEDIUM POWERS. 

 — In dealing with the subject of low power photo-micrography 

 in the June number of "Knowledge" we explained that 

 the size of the images produced was the result of considerable 

 camera extension '" one hundred and sixty-three centimetres," 

 and in the last issue of the journal it was shown how the 

 magnification could be still further auguieiited, by the use of 

 an eyepiece "ocular" although at the expense of very con- 

 siderable restriction of the 

 field. If the two illustrations, 

 photo - micrographs of the 

 proboscis of a blow fly, one 

 magnified eighty diameters 

 and the other "a portion" 

 three hundred and twentx 

 diameters, be carefully 

 examined, we fail to find 

 any difference in the amount 

 of detail present in either. The 

 cjuestion, therefore, naturally 

 arises as to what is the use 

 of this extra and great ampli- 

 fication ; what does it do ? and 

 is it necessary or desirable '- 

 To answer this question several 

 things have to be taken into 

 consideration, such as the 

 resolving power of the 

 objective, and the magnification 

 that may be necessary in order 

 that the eye may be able to 

 clearly discern the structure 

 resolved. .As the primary use 

 of a microscope is to enable 

 the observer to see distinctly 

 the minutest details in the 

 image of the object under 

 observation, a degree of 

 enlargement that will ensure 

 this would appear to be all that 

 is necessary, unless from indi- 

 vidual requirements or photo- 

 graphic purposes it be desired 

 to go beyond it. In order to 

 arrive at the conditions 

 requisite to get the most out 

 of our instrument, we must 



first of all consider the objective, with regard to its 

 ability to resolve fina structure. Now it is well known 

 that the resolving power of any lens depends upon its 

 aperture, and that of the one inch objective employed 

 in photographing the images of the proboscis was 

 found to be -2 N..A., and this theoretically should possess 

 a resolving power for the brightest part of the spectrum 

 visually of nineteen thousand. In other words, the lens 

 should be able to separate lines or structures as close 

 together as the one nineteen-thousandth of an inch. It 

 only remains then to magnify the image sufficiently 

 that the eye may be able to see the details readily when 

 viewed under proper conditions. It is generally considered 

 that the normal eye is able to distinguish lines as close 

 together as the one two hundred and fiftieth of an inch 

 when viewed at a distance of ten inches from it. So 

 that in order to take full advantage of the resolving 

 power of our lens we should ha\e to enlarge the image 

 seventy-six times, in this case, to fulfil the required conditions. 

 As the magnification was eighty diameters nothing further 

 should be gained by increasing it, unless, of course, it were 

 found not sufficient in individual cases. It has been stated 

 that the limit for best definition is reached by employing as 

 the degree of magnification " in diameters." the value obtained 

 by multiplying the N.A. of the objective by four hundred. So 



I'lGl'kL j^Z. 

 Stomata in Cuticle of Mistletoe X 200 diameters. 



Photographed with a Zeiss 16 mm. Apochromatic Objective and 

 a four-inch Projection Ocular. 



that in the above case in which the N..-\. = -2 we should get 

 400 X -2 = 80 diameters, and any further increase beyond 

 this would result in an inferior image. In considering, then, 

 the question of photomicrography with medium powers, it is 

 not so much the increase in the initial magnifying power of 

 the objective that concerns us, as the gain due to their larger 

 aperture. As an example we may take the illustration to 

 this article, which was photographed with a Zeiss sixteen 

 millimetre apochromat with a four-inch projection ocular. As 

 far as size of image is concerned, this could have been 

 obtained by using a higher eyepiece with the lower power, 

 but the sixteen luilliMietre has an aperture of -3 N.A. as 

 against -2 N..\. in the former 

 case, which means that the 

 resolving power is half as much 

 again with the sixteen milli- 

 metre lens compared with 

 that of the twenty-four niilli- 

 nietre one. We thus see that 

 the aperture is the all-impor- 

 tant considcr.-ition ; that mere 

 magnification beyond the 

 degree necessary for obser- 

 vation becomes useless, but 

 as the initial size of the 

 image increases the higher 

 the power of the objective, a 

 lower eyepiece can be em- 

 ployed with advantage. The 

 illustration (Figure 382), 

 taken with a Zeiss sixteen 

 millimetre apochromatic ob- 

 jective and a four-inch projec- 

 tion ocular, was magnified four 

 hundred diameters. The 

 negative was made on an 

 Imperial N. F. plate of a 

 speed of two hundred and 

 twenty-five H. and D. An 

 exposure of ten seconds was 

 given, using a deep orange 

 screen, the specimen being 

 stained blue. The image was 

 developed as before with pyro 

 and soda. 



and D., the subject a 



EXPOSURE TABLE 

 FOR SEPTEMBER. — The 

 calculations are made with 

 the actinograph for plates 

 of speed two hundred H. 

 near one, the lens aperture F. 16. 



Remarks. — If the subject be a general open landscape, take half 

 the exposures given here. 



ZOOLOGY. 



By Professor J. Arthur Thomson, M.A. 



FORMATION OF HABITS.— One of the most interesting 

 kinds of modern work in natural history is the study of habit- 

 forming. Mr. .'\sa A. Schaeffer has been recently experimenting 

 with frogs in this connection. Individuals of three different 



