312 



KNOWLEDGE. 



August, 1913. 



means of migration. This family is divided into about six 

 genera. 



The Gamasidae are the largest family of the three, containing 

 about twenty genera. A number of papers have been published 

 in the British Isles on the Gamasoidea in natural history 

 and microscopical journals. Mr. Michael has given two, if 

 not more, very interesting ones on the internal anatomy of 

 Gamasidae and Uropoda; also one on the life-history of 

 Gamasidae, and in 1888 described one new species. Dr. 

 George has also described and recorded some rare and 

 interesting species in The Naturalist from time to time — 

 three species in particular, of the genus Epicrius. But there 

 is no monograph on any of the three families yet, neither is 

 there a Tierreich. 



Figures 340-342 will help to convey the general appear ■ 

 ance of each family. But, as Mr. Banks says, the Der- 

 manyssidae and the Gamasidae are so closely allied by 

 structure that the parasitic habits are the best character for 

 separation. 



C. D. Soar, F.L.S., F.R.M.S. 



QUEKETT MICROSCOPICAL CLUB.— May 28th — 

 T. A. O'Donohoe read a paper on " Minute Structure of 

 Coscinodiscus asterotnphalus, Pleurosigma angulatum, 

 and P. balticum." The object of the paper was to prove 

 that the " black dot " is the correct image of diatom structure. 

 Photomicrographs, mostly at X 4000, were shown of fragments 

 of the diatoms mentioned. When the edge of the fracture was 

 sharply focused the transparent silex was shown as white 

 and the perforations as black. On slightly raising the 

 objective the silex was rendered as black and the perforations 

 as white. It was therefore thought to be justifiable to relegate 

 the " white dot " images of diatoms to the abode of Mr. 

 Nelson's " ghosts." 



June 24th. — E. M. Nelson sent a note describing Koristka's 

 new loup. 



H. Sidebottom, contributed a paper on " The Lagenae of 

 the South-West Pacific." This is part two of a paper 

 published in the April 1912 issue of the Club Journal. 

 There was some discussion as to the significance or use 

 of the very beautiful markings and decorations found 

 in this and other groups of organisms. The President could 

 not conceive it possible that the presence or absence of a 

 minute projection on a sponge spicule, for instance, could 

 make any difference whatever to the organism, especially as 

 in the case of the sponge, the spicule is buried in the general 

 protoplasmic mass of the animal. In the case of f oraminifera 

 the markings are invisible during life, as they are concealed 

 under the usual gelatinous mass of exterior protoplasm. 



E. M. Nelson described " A new method of measuring the 

 magnifying power of a microscope." The " constant " of an 

 eyepiece with a given tube-length is determined. This 

 " constant " is found by first determining by any of the usual 

 methods the combined magnifying power of that eyepiece on 

 the given tube-length with a medium power objective, such as 

 half-inch. Second, measure the exact diameter of the field by 

 means of a stage micrometer. The product of these two 

 quantities is the " constant " of that eyepiece with the 

 given tube-length. Example — objective one-third of an 

 inch, eyepiece, compensating X 8, tube-length 170 milli- 

 metres, measured magnifying power X 280, measured 

 field 0-023 inch, product is 6-44, which is the "constant" 

 of that eyepiece for 170-millimetre tube. The power of 

 any other objective with this eyepiece and tube-length 

 can be determined by measuring the diameter of its 

 field with the stage micrometer. The magnifying power 

 will obviously be the eyepiece " constant " divided by the 

 diameter of the field (an objective of varying power with a 

 negative front cannot be measured in this manner). A 

 number of examples were given. The " constant " may also 

 be employed for ascertaining the total magnification for any 

 tube-length. All other conditions being the same, the total 

 magnification will be proportional to the tube-length used. 



PHOTOGRAPHY. 



By Edgar Senior. 



USE OF GLYCERINE IN DEVELOPING PLATINUM 

 PRINTS. — As mentioned in last month's notes on platinum 

 printing, glycerine is of use in the development of these prints, 

 as when added to the developer it so reduces its rapidity of 

 action as to enable local development to be carried out with- 

 out any difficulty. As the sensitive salts with which the paper 

 is coated are almost insoluble in glycerine, this may be 

 brushed over the surface of the exposed paper without risk of 

 injury to the print. A mixture of glycerine and developer 

 may then be applied with a brush and an over-printed portion 

 of the subject may be retarded by the application of a 

 weaker developer, or one containing more glycerine, until the 

 other parts of the image are sufficiently strong. This method 

 of treatment lends itself particularly to the production of 

 vignetting effects on account of the great softening of the 

 edges that can be obtained. The strength of the solution 

 employed by different workers varies, but the following will be 

 found to answer well : — 



Oxalate of Potash 

 Water 

 Glycerine (pure) 



1 ounce 

 3 ounces 

 1 ounce 



This will form a stock solution which for use may be taken 

 in the proportion of one part mixed with an equal volume of 

 water. If preferred, the developing salts may be used in 

 place of the oxalate, as with a little practice it is quite easy to 

 control development with either. When the desired result has 

 been obtained the prints are placed for the required time in 

 each of the three acid baths, and finally washed and dried, 

 intensifying under-exposed platinum prints with gold. 



Some years ago Mr. Alfred W. Dolland published a 

 method of strengthening an under-exposed platinum print by 

 means of a solution of gold chloride applied with glycerine. 

 The writer tried this at the time with perfect success. The 

 procedure consists in applying glycerine to the finished print 

 and then spreading a solution of gold chloride by means of a 

 piece of cotton-wool or a brush, the strength of the gold 

 solution which we used being fifteen grains in fifteen drachms 

 of distilled water. The print rapidly gained in strength, 

 assuming a fine bluish-black colour, and when the action had 

 proceeded far enough the print was washed, and an ordinary 

 amidol developer applied for a few minutes in order to reduce 

 any remaining gold chloride to the metallic state, when a final 

 washing completed the operation. We have quite a number 

 of prints that were treated in this manner, which appear 

 as fresh as when first produced, and certainly the colour in 

 many cases produced a more pleasing effect. Platinum prints 

 may also be intensified by a method, due to Baron Hubl, of 

 depositing further platinum upon them. In order to employ 

 this method the following solutions have to be made up : — 



Sodium Formate ... 

 Water 



Platinum Tetrachloride 

 Water 



48 grains 

 1 ounce 



10 grains 

 1 ounce 



For use fifteen minims of each of the above are taken and 

 made up to two ounces by the addition of water. The print 

 is placed in a flat dish and the solution is poured over the 

 dish, which is kept rocking until the desired effect is seen in 

 the print, when the latter is washed and dried. 



WARM TONES IN PLATINUM PRINTS.— Various 

 substances have from time to time been recommended as 

 additions to the developer in order to vary the colour of a 

 platinum print, but the one most generally employed is 

 mercuric chloride, as by its means, together with variations in 

 the temperature of the developer, tones ranging from warm 

 black to sepia are readily obtainable. Numerous formulae for 



