464 



KNOWLEDGE & SCIENTIFIC NEWS. 



[June, igo6. 



Conducted by F. Shillington Scales, r..A., f.r.m.s. 

 Elementary Photo-micrography. 



{Coii/inr/cJ from page 440.) 



The plate used in photo-micrography should be, 

 therefore, an orthochromatic one, it should be of 

 medium rapidity in order to get contrast and latitude 

 of exposure, and it should be " backed " to save re- 

 flections. There are many such plates on the market, 

 the Edwards, Barnet, and Ilford plates are all well 

 known; the first is, perhaps, the most popular, but ac- 

 cording to some very interesting tests made by Dr. 

 Spitta, the " Isochrom " plate of the Ilford Company 

 has a wider range, and is, therefore, superior. The 

 Lumiere plate is rather more expensive, and the Cadett 

 plate has the largest range of all, but is awkward to 

 develop on account of this very sensitiveness. It is 

 w-ell to make a practice of using one kind of plate and 

 to adhere to it except for some special reason. 



The necessarv screens will be two yellow screens, one 

 of them dense, two blue screens of the same descrip- 

 tion, and a signal-green or pot-green screen. Gifford's 

 F line screen is a very useful one, whether made of 

 glass and gelatine or of signal-gresn glass and mala- 

 chite green and glycerine, which passes more light. 



The question of exposure is a difficult point to give 

 hints upon, as it varies enormously with the light, the 

 plate, and the objective and magnification, and I am 

 afraid nothing but trial and experience will be of ser- 

 vice. Two things may be said, however; firstly, that 

 orthochromatic plates of medium speed have extra- 

 ordinary and unexpected latitude in this respect, and, 

 secondlv, that in case of over or under exposure it is a 

 fairly safe rule to boldly halve or double the exposure, 

 as indicated by the state of the plate on development, 

 and not to try intermediate differences of time. Per- 

 haps a few examples of exposures taken from my own 

 record book may be some little guide to an absolute 

 beginner. Thus, with lamp-light and a Herschel 

 auxiliarv condenser, in addition to the sub-stage 

 condenser, without screens, with a projection eye- 

 piece X3, and a moderate camera length, a i-inch objec- 

 tive required 45 seconds exposure, and a i-inch objec- 

 tive about three minutes; whilst with the oxy-hydrog-en 

 Sight and a Conrady achromatic and aplanatic con- 

 denser, a i-inch objective required 10 seconds, and a 

 |-inch 4S seconds. The use of screens would double 

 or treble these exposures, and the nature of the object 

 may also make much difference. 



The develop?rs differ somewhat from ordinary photo- 

 graphy, as we require not soft graduations, hut sharp 

 blacks and whites. The best developer 1 know of for 

 this purpose is metol and hydroquinone, but for par- 

 ticulars as to the use of this I must refer the reader to 

 ordinary books on photography, merely observing that 

 development should be carried beyond the stage re- 

 quired for ordinary work. Whatever developer is used, 

 however, it should be mastered and adhered to as a 

 general rule. Intensification and reduction are valuable 

 aids in modifying or improving otherwise unsatisfac- 

 torv negatives. 



The best paper is a Bromide paper, amongst which 

 Nikko " is excellent for our purpose. Printing-out 

 papers are less satisfactory, especially for reproductive 

 work for the press, but are capable of being watched 

 and humoured rather more, and are useful also for 

 rough trials. 



These notes have already run to a greater length 

 than I originally intended, but I have tried to keep 

 them as elementary as possible, and to refrain from 

 going into too much detail, though it has been a con- 

 stant temptation to enlarge on the many important 

 points dealt with. I hope I have, however, explained 

 the essential points which a beginner needs to know.* 



R^oyal Microscopical Society. 



April 18— J. C. Karop, I-;sq., MT^.C.S., \'ice-Presi- 

 dent, in the chair. Dr. Hebb exhibited and described a 

 simple and effective form of apparatus for obtaining 

 blood for bacteriological examination and cultivation. 

 He also showed some cultures of bacteria on blood 

 serum and agar, which were preserved in formalin. The 

 cultures were killed, and at the same time mounted, by 

 pouring into the test-tube 10 per cent, formalin on the 

 top of which was placed a mixture of melted paraffin 

 and vaseline. When cool, this formed an air-tight and 

 stable cylindrical stopper. Dr. Hebb remarked that 

 the method was not adapted for all cultures, as some 

 were dissolved off the surface by the preservative fluid. 

 Dr. Hebb also exhibited some test-tubes containing 

 sterilised nutrient broth plugged in the same way. The 

 object of the plug was to allow the tubes to be trans- 

 ported from place to place without damage to or loss of 

 the medium. To remove the plug it was onlv necessary 

 to warm the tube. .A series of coloured lantern slides 

 of botanical sections by Mr. Flatters, of Manchester, 

 were shown. 



Quekett Microscopical Club. 



April :;o — the President in the chair. The Hon. Edi- 

 tor, Mr. F. P. Smith, brought forward two papers. The 

 first, " On the Spiders of the DiphcepJtalus Group," 

 concluded the revision of the British species of the sub- 

 family Erigoninje. The second paper was a catalogue 

 of the literature dealing with Erigonine spiders. 



Mr. H. Taverner, F.R.M.S., gave an account of the 

 methods he employed in stereo-photomicrography. The 

 axial rays were found to be detrimental to the forma- 

 tion of a stereoscopic imag^e, and after considerable ex- 

 periment a form of Davis's shutter is now employed, 

 fitted above the objective and able to be shifted by screw 

 adjustment to a maximum of 4 mm. either side of the 

 optic axis. To obviate cutting the prints for mounting, 

 a repeating back is used and the image received on the 

 left half of the plate, with the aperture of the stop to the 

 left of the optic axis, and the second exposure on the 

 right half of the plate with stop to the right of the optic 

 axis. The diameter of the aperture was usually 2i 

 mm., with its inner edge not more than i mm. from the 

 optic axis. 



The Hon. Secretary read a paper on the same subject 

 by Mr. ^^'. R. Dollman. Only low powers are used — 

 photo objectives with focal lengths of from two to six 

 inches being employed. A semi-circular shield is placed 

 against the diaphragm between the combinations and 

 rotated through 180° between the two exposures. 

 .'\cetylene is used as illuminant. 



Mr. J. Rheinberg, F.R.M.S., read a long and techni- 

 cal paper dealing with " Stereoscopic Effect, and a 



•The first instalment appeared in" Knowledge," for November, 



