August, 1912. 



KNOWLEDGE. 



319 



in choosing his scheme of lighting to view the object from just 

 over the top of the camera and looking at it through a piece 

 of rough black paper rolled up to form a viewing tube when 

 held close to one eye. " j, ^ Lambert, M...\., F.R.P.S. 



PHOTOGRAPHY. 



By EdG.^R Sli.NIOR. 



THE USE OF PROJECTION OCULARS IN PHOTO- 

 MICROGR.-\PHY. — In our article on low power photo-micro- 

 graphy in the June issue of "Knowledge," we showed 

 examples taken bv means of the objectives only, making use 

 of the extreme camera extension available, "one hundred rind 

 sixty-three centimetres," 

 in obtaining the magnifi- 

 cations given. If, how- 

 ever, it be desired to 

 still further increase thi 

 size of our image, in 

 order to render visibN 

 detail too small to hv 

 readily seen by the 

 unaided eye, then resort 

 must be made to the 

 use of an ocular ("eye 

 piece ") as well. A 

 deal of diversity ot 

 opinion exists as to 

 whether the ordinary 

 oculars employed foi 

 visual observation 

 should be used in 

 photographing. Wr 

 ourselves have obtained 

 excellent results with 

 them. For great mag- 

 nitication. the compen- 

 sating oculars used in 

 conjunction with apo 

 chromats will be found 

 to work well with modern 

 achroniats. There can 

 be no doubt, however, 

 that wherever possible it will be advisable to employ a 

 projection ocular, for although primarily intended for use 

 with apochromatic objectives, they give excellent results 

 with achromats. This eyepiece consists of a collective 

 lens, and a triple projection combination at the other end of the 

 tube, and the former, " when the specimen is in focus." forms, 

 in combination with the objective, an image of the object in the 

 plane of the eyepiece diaphragm. The edge of this diaphragm 

 is then sharply focused upon the ground glass screen by 

 means of the focusing collar in which the projection lenses 

 are set. If attention be paid to this, an exquisitely sharp 

 image should result. One drawback to these oculars lies in 

 the great restriction of the field, so that it is impossible to use 

 themforany object at all large. The illustration (see Figure 356) 

 is from a negative taken with a one-inch objective of Messrs. 

 James Swift & Son, together with a four-projection ocular of 

 Messrs. Zeiss. The magnification due to the objective alone 

 was eighty diameters, and this multiplied by four, '" the value 

 of the ocular," gives a total magnification of three hundred 

 and twenty diameters, due to the combination of objective and 

 eyepiece together. The exposure given was six minutes, 

 calculated from the square of the increased magnification due 

 to the eyepiece. The plate employed was an Imperial N.F. 

 of speed two hundred and twenty-five H and D, and a green 

 screen was used, as before, to give greater contrast. The 

 negative was developed with pyro and soda developer, contain- 

 ing one grain of potassium bromide in each ounce of solution. 



PROCEEDINGS OF THE OPTICAL CONVENTION. 

 I.— COLOUR PHOTOGRAPHY.— A paper entitled "The 

 micro-spectra method of colour photography," by Julius 

 Rheinberg. F.R.M.S., deals with a means of producing 



Figure 356. 



Pholographtd with Swift & .Son's 1-inch objective and 

 The original wa.s magnified 320 diameters, am 



images in the colours of the objects by photography, without 

 the use of colour screens, coloured pigments or particles, 

 or any device of this nature. It is a one plate, one exposure 

 method, the image, which is black and white, being developed 

 in the ordinary manner. The principle is one of optical 

 synthesis, in which the blending of spectrum colours produces 

 the sensation of white light upon the retina, when if by any 

 means the proportion in which the colours are mixed be 

 altered, the resulting effect is no longer of white but of colour. 

 The method that has been adopted to carry out practically the 

 principle involved, is most ingenious. It is well known that if a 

 narrow slit in an otherwise opaque screen be illuminated by 

 white light and an image of it focused upon a screen, the inter- 

 posing of a prism results in the formation of a spectrum 



image of the slit, but 

 if instead of a single 

 slit a number be em- 

 ployed, then a number 

 of spectrum images re- 

 sults. The authors of 

 this process of colour 

 photography have taken 

 advantage of this in 

 designing a screen, 

 having four hundredslits 

 per inch, the ratio of 

 which to the opaque 

 spaces is in the pro- 

 portion of one to three, 

 and by the use of a 

 prism with small dis- 

 persion they have been 

 able to obtain a 

 distinct and separate 

 spectrum with each slit. 

 The photographic ob- 

 jective used in the 

 camera magnifies the 

 images of the slits 

 four times. The result 

 is a series of regularly 

 repeating spectrums 

 side by side, without 

 any intervening gaps 

 between them, and practically without any overlap, of one 

 hundred per inch. And since the individual colours cannot 

 be separately distinguished the sensation of white light results 

 from their optical combination. In taking a photograph, the 

 image of the coloured object is projected on to the line screen 

 by means of a lens ; the line screen and image are then 

 focused on to the ground glass screen of the caniera by means of 

 a second lens with the analysing prism in front of it. The plate 

 employed in taking the negative must, of course, be a pan- 

 chromatic one. From the negative obtained, a positive 

 transparency is made, and this, when placed in the camera 

 " in the exact position occupied by the negative and 

 illuminated by white light," will, by means of the deposit 

 of silver forming the image, so remove from the white light 

 those constituents not active in taking the negative image 

 that the remainder will impart the colours necessary. If, for 

 instance, red and green light h.id impressed themselves in the 

 negative, these parts would be transparent in the positive, and 

 the red and green would optically form yellow. This process of 

 colour photography appears to be a most fascinating one. The 

 paper is full of the most interesting problems, and the authors 

 deal at some length with the various difficulties encountered. 

 A normal spectrum such as is given by a diffraction grating 

 was desirable, and a prism to give this result was computed by 

 Mr. Conrady, of Messrs. Watson & Sons. 



II.— PHOTOGRAPHIC LENSES.— In a paper on the 

 transmission of visible light by photographic lenses, by R. W. 

 Cheshire, B.A.. the author occupies himself with some experi- 

 ments carried out at the National Physical Laboratory, on the 

 loss of light due to reflection and absorption by the glass 

 composing the objectives. For a surface that has not been 



Portion of the Proboscis of a Blow-fly. 



lar of Ze 



-inch projection < 

 eproduced half-si 



