February. 1915. 



KNOWLEDGE. 



the gap. A lens may be ver^- well corrected, in the sense 

 that some are described as semi-apocliromatic, but it 

 does not give it those exact quahties which the apo- 

 chromatic objective should possess. That British makers 

 are able to comply with the conditions is beyond question, 

 and it is to be hoped, now that the Continental supply is 

 stopped, users in this countrv will realise that the production 

 of the best optical firms here is Ukely to be equal in every 

 respect to that of Continental origin. 



THE CARE OF A MICROSCOPE.— It is no uncommon 

 thing, particularly in laboratories, to see an ordinar\' 

 duster or some other dirtj' cloth used for cleaning lenses 

 or other optical parts of a microscope. This is a most 

 pernicious habit from every point of \'iew. Nowadays the 

 polish on optical surfaces is of a high order, and anything 

 which tends to destroy this polish interferes \\'ith the per- 

 fection of the image, and causes loss of light. If hnen is 

 used for cleaning purposes, it should be old, and must be 

 thoroughly wEished before use, so that it is cleansed from 

 all dirt and grit. A better method, particularly when 

 working ^\•ith oil-immersion objectives, is to use Japanese 

 rice paper, which is now easily procurable, and is very soft 

 and clean. A small piece of this may be torn off, and the 

 lenses carefully wiped with it and the piece thro^vn away. 

 This will be found less expensive, less troublesome, and more 

 efiective than using iinen or cloth of any description. Any 

 optical parts, even if temporarily out of use, should not be 

 left uncovered on the working bench. For this purpose an 

 ordinary glass mav be inverted and placed over the lens, or 

 a small bell jar, purchasable for a few pence, may be used. 

 The brass cases for objectives should always be laid on the 

 bench with both bottom and top inverted, so that no dust 

 can enter. 



THE QUEKETT MICROSCOPICAL CLUB.— The five 

 hundred and fourth ordinan,- meeting of tlie Quekett 

 Microscopical Club was held on Tuesday, January- 26th, 

 at 20, Hanover Square, W. ; the President, Professor 

 Arthur Dendy, D.Sc, F.R.S., in the chair. Three 

 gentlemen were elected members, and four others were 

 nominated for election. The names of officers nominated by 

 the Committee for the ensuing year were read, being sub- 

 stantially the same as at present, with Professor Dendy 

 again as President. The members proposed the names of 

 gentlemen to serve on the Committee to fill the vacancies 

 caused by the retirement of senior members. The ballot 

 will take place at the annual meeting. 



Vice-President Professor E. A. Minchin, M..\., F.R.S., 

 then gave a paper, " Notes on Flea Anatomy." He said 

 that, although the main purpose of his researches was to 

 trace the development of the Trj-panosomes found in the 

 rat flea, ha\'ing with the help of a friend dissected about 

 one thousand seven hundred fleas, it was ine\-itable that an 

 intimate knowledge of the minuter structure should be 

 acquired. He then described the instruments and the 

 methods employed, and gave a full account of the anatomy 

 and histolog3' of the internal organs, comprised under the 

 following heads : (1) Notes on the abdominal nervous 

 system, sho\ring the curious difference between the male and 

 the female ; (2) on the male reproductive system ; (3) on the 

 female reproductive system ; (4) on the stellate muscle- 

 cells of the oesophagus. The lecture was of a most interesting 

 character, especially to the audience to whom it was 

 addressed, appealing particularly to microscopists. It was 

 illustrated by lantern diagrams thrown on the screen, and 

 by a series of beautiful micro-preparations exlubited under 

 microscopes on the table. These slides Professor Minchin 

 has presented to the Club, and they will be added to the 

 cabinet for the future use of members. Professor Dendy 

 made a few appreciative remarks, and proposed a vote of 

 thanks to the lecturer, which was heartily accorded by 

 acclamation. 



The next ordinary meeting will take place on Tuesday, 



Februarv 23rd, when, after the usual business of the annual 

 meeting. Professor Dendv will deliver the Presidential 

 address, his subject being " The Biological Conception of 

 Individualitv." 



T B. 



PHOTOGRAPHY. 



By Edgar Senior. 



THE ELIMINATION OF THE SOLUBLE SALTS 

 .\ND "HYPO" FROM THE GEL.\TINE FILM.— In 

 these days of huny- and scurry there is a growing tendency 

 to give but little attention to the fixing and washing of 

 negatives and prints. With the idea of saving time, they 

 are often imperfectly fixed, and still more often imperfectly 

 washed. Man^' workers appear to lose sight of the fact 

 that it is false economy to withdraw either negatives or 

 prints too soon from the fi.xing bath, as, when properly 

 fixed, they can be freed from soluble silver salts and " hypo " 

 much more rapidly than otherwise would be the case. 

 .\s already pointed out in these columns, it is a good plan 

 to use tvvo fixing baths, the second one ensuring a complete 

 conversion of the silver salts into the soluble variety', which 

 readily diffuse out in the washing water. Of the various 

 processes through which a negative passes it can be said 

 that there are few of greater importance than thorough 

 washing, for unless this be properly done endless trouble 

 will arise in any after treatment that may be found necessary. 

 Then, again, although there are many excellent washers on 

 the market, the amateur often makes use of some domestic 

 utensil, which is quite unfitted and unsuitable for the pur- 

 pose. In any case, whatever kind of appliance be employed 

 for washing, it should be borne in mind that, as the " hypo " 

 and silver salts leave the film, their weight causes them to 

 sink to the bottom of the vessel, whence they should be 

 rapidly syphoned away. 



FUGITIVE SILVER PRINTS.— It is often stated that 

 silver prints made years ago were more permanent than 

 many produced of late, some of which show signs of fading 

 after a few months. As an explanation, the class of negative 

 used has generally been taken into account, as in the early 

 days the negatives employed were much denser, the printing 

 paper (albuminisedl was salted in a strong bath, and the 

 printing carried to a greater depth and more gold deposited 

 in toning. Then, again, the washing was more thorough, 

 and performed more quickly. Long wasliing degrades the 

 brilliancy of the prints. The old method of working was to 

 wash the prints in large dishes alternately in cold and warm 

 water, well draining between each change, the prints by 

 this method being practically freed from " hypo " in about 

 twentj' minutes. Then the prints were made from denser 

 negatives upon paper sensitised with a strong silver bath, 

 and fixed in a stronger fixing bath — in many cases in 

 two fixing baths — and then quickly washed ; and to 

 treatment of this nature has been ascribed the secret of 

 their permanence. It has also been found that prints which 

 are thoroughly fixed, but imperfectly washed, are more 

 permanent than those that have received a thorough wash- 

 ing, but have been imperfectly fixed. There is httle doubt, 

 however, that the mounts employed have, in many cases, 

 been the cause of fading : among prints in the possession of 

 the writer, made upon albuminised paper, some mounted and 

 others unmounted, it is usually the mounted ones that have 

 faded the most, ^^^len gelatino-chloride paper was first 

 introduced we were told that, in a very short time, it would 

 entirely- displace albuminised paper, and that the prints 

 made upon it would be permanent ; the latter claim, 

 however, has not been realised in practice. Prints made 

 upon gelatino-chloride papers when carefully worked do 

 appear to be more permanent than those made upon the 

 older albuminised papers, but it would be incorrect to call 

 them permanent. The degree of permanence appears to 

 depend to a considerable extent upon the method of toning 



