434 



KNOWLEDGE. 



November, 1913. 



forms, even under this magnification, it will be seen at once 

 how difficult is the task of photographing the still more minute 

 secondary structure— six or more faint dots within each cellule. 

 Nothing is more fitted to try the worker's mettle ; indeed, one 

 may say from one's experience in diatom structure, nothing 

 else is so fitted. There still remains to try the mettle of the 

 process worker and others in preparing the blocks. 



The drawing of the finer structure of this diatom by Messrs. 

 Nelson and Karop, published in The Journal of the Quekett 

 Club for 1887, shows a few cellules only. Although professedly 

 drawn to a scale of one thousand two hundred diameters, 

 each cellule appears as of twice the sue of those in the present 

 prints, taken at one thousand nine hundred. One can question 

 neither the good faith of the artist nor the real size of the 

 details in the valve, yet the discrepancy can be easily accounted 

 for by supposing that Mr. Karop found them almost impossible 

 to make plain to the eye on so small a scale. This is the 

 difficulty the present writer fears now in his own prints, though 

 trusting to the skill of all connected with the production of 

 " Knowledge." 



In the drawing in question the six or seven dots within each 

 cellule are rendered quite pale, a characteristic feature of this 

 diatom, and one making them so difficult to photograph. The 

 boundaries are formed by a series of similar dots, but black. 

 One does not like to differ from such eminent observers 

 as Messrs. Nelson and Karop, at the same time one 

 cannot help feeling that they are due to interference 

 from the under-structure of hexagons. Something of like 

 appearance occurs in Figure 513 of the present article, 

 yet on using the largest aperture of the substage con- 

 denser they disappear and the dotted membrane is seen 

 to be continuous over the whole valve. There would be 

 work for the owners of oil immersions during the winter 

 months to solve this problem and then send the results to 

 " Knowledge " — with the consent of the editors, of course. 

 The evidence of a continuous membrane is not lacking in some 

 of the coarser discoid forms. We have already seen that in 

 Triceratium favus and Coscinodiscus asteromphalus there 

 are structureless parts from which the kind of woven material 

 forming the secondary structure springs. In one species of 

 Coscinodiscus, however, from the Nottingham deposit, there is 

 nothing of this ; instead a uniform perforated membrane, 

 stuck upon little bosses some distance above the hexagons, 

 spreads over the whole valve. 



Coming to the figures now, Figure 510 is of the same valve 

 from which Figure 411 of the last article was taken at one 

 thousand nine hundred diameters. Unfortunately the par- 

 ticulars underneath the print say two thousand seven hundred 

 and fifty, but this is a mistake. In some manner the 

 particulars of four hundred and ten and four hundred and 

 eleven have become transposed. Figure 410 should read 

 two thousand seven hundred and fifty diameters and 411 

 nineteen hundred. The next, Figure 512, is from an 

 undoubted four-rayed normal form of A. kittonii, and it will 

 be seen at once how minute are the cellules as compared with 

 Figure 510. Figure 514, taken at one thousand nine hundred 

 diameters from the centre of the same valve, shows the 

 secondary structure, but so minute as almost to require 

 another lens to see it. In Figure 509 we go back to Figure 

 411 of volume XXXV in order to compare the size of the 

 cellules with Figure 514. The contrast between the two will 

 then speak for itself. 



Figure 508 is from an eight-rayed valve, a very beautiful 

 variety. A photo-micrograph of the same variety of this 

 species appears in the two editions of the Dallinger-Carpenter, 

 taken at two hundred and seventy diameters, by Mr. E. M. 

 Nelson, with an inch objective under dark-ground illumination. 

 Seen on either the first or second plate it looks innocent 

 enough, until one attempts the same performance with an inch 

 objective. Believe one who has tried, and must confess 

 to a dismal failure or failures in consequence. Figure 513 

 is from the centre of Figure 508, which, with Figure 514, 

 the writer hopes may be taken as a fitting supplement to Mr. 

 Nelson's print. He hopes also that he has now made his 

 amende honorable, as really having done what he only 

 boasted of doing before. 



The varieties of A. kittonii range from three rays to nine. 

 The centre valve in Figure 511 is a five-rayed form, and is 

 curious otherwise as showing that Nature's freaks, such as 

 Two-headed Nightingales and Siamese Twins, are not con- 

 fined to complex organisms. At the end of each process in 

 the normal form, under high powers, a little bulb of crystal can 

 be seen, in shape like those used to light the carriages in the 

 tube railways. They are just indicated in the print, but at 

 the end of one process two appear in place of the usual one. 

 The specimens in this mount are from the west coast of 

 Africa. Ten in number consist of the four-rayed forms, one 

 of five, yet the secondary structure is so minute as not to offer 

 the slightest signs of resolution under an aperture of 1 • 30 

 N.A. Notice also the different shape of the processes from 

 Figures 512 and 508. 



PHOTOGRAPHY. 



By Edgar Senior. 



GLAZING SILVER PRINTS.— When it is desired to 

 impart a high degree of glaze to P.O. P. or bromide prints 

 whcih are made upon a glazed surface paper, the prints after 

 washing should be placed in a solution of some hardening 

 substance, such as ordinary alum, chrome alum, or formalin. 

 Especially in warm weather is this treatment necessary, since 

 the gelatine surface often becomes very soft and liable to 

 adhere to the surface used for squeegeeing upon, whatever 

 precautions may have been taken to avoid it. If alum is 

 employed as the hardening agent a five per cent solution 

 should be employed ; formalin, however, is to be preferred in 

 many respects, as alum is not always successful in its action. 

 The strength of the formalin solution should be about one 

 ounce to ten or twenty ounces of water, and the prints should 

 be allowed to remain in for five or ten minutes and then 

 washed in several changes of water. With regard to the 

 materials to be used for squeezing the prints upon, these may 

 be either glass, ferrotype plates, or celluloid, the latter two 

 being the least likely to give trouble from the prints sticking. 

 If glass be used it requires to be very thoroughly cleaned, and 

 must be soaked for some hours in either of the following : — 



Nitric acid 5 ounces 



Water 20 



or 



Potassium bichromate ... 1 ounce 



Water 30 ounces 



Sulphuric acid ... ... 1 ounce 



The bichromate should be dissolved and the solution made 

 perfectly cold before the sulphuric acid is added, this latter 

 being introduced very gradually, stirring well all the time. 

 As this mixture is very corrosive, great care is required in 

 handling it. After the glass plates have been soaked for 

 several hours, they should be well washed in water and 

 allowed to dry, when they are polished with French chalk, this 

 being dusted on to the plate and well rubbed over and 

 finally dusted off again. Or in place of the French chalk a 

 solution of beeswax in benzol or turpentine may be applied 

 with a piece of rag or a tuft of cotton-wool, and the surface 

 afterwards polished with a clean cloth. The following formula 

 will give a good solution for the purpose : — 



Beeswax 15 grains 



Turpentine 1 ounce 



The clean glass, ferrotype plate, or celluloid should have a 

 little of this well rubbed over the surface and then polished 

 off. The material being ready, the prints are taken from the 

 washing water and laid face downwards, and then squeegeed 

 into contact and allowed to become thoroughly dry before any 

 attempt is made to strip them, and on no account must the 

 drying be at all accelerated by heat, or the prints will be 

 difficult to remove from the support. Sometimes difficulty is 

 experienced in getting them off, but this can be prevented by 

 drying the prints first and then rewetting them ; and if care be 

 taken to have the surface of the material thoroughly well 

 cleaned and prepared the prints should almost leave by them- 

 selves, especially so when ferrotype plates are used. 



