KNOWLEDGE • 



[June 23, 1882. 



solution must be kept in constant motion, not violently, or 

 airbubbles will be caused. In a short time the picture 

 will appear, but the development must be continued until a 

 strong image is seen when the plate is examined by trans- 

 mitted light Experience witli a wet plate is of little use 

 here, as the appearance of the image is quite diflTcrent. 

 The development miist be continued until there is an 

 appearance of gi-eat density, and the back of the plate 

 Wing examined, the picture should to some extent be visible 

 there. Practice with a few plates will be worth more than 

 pages of written description. 



Any make of gelatine plates may be developed with 

 either of the solutions named, but in some cases it is 

 atlvisable to follow the directions to be found with 

 each packet of plates. 



If the picture do not appear in moderate time, the ex- 

 posure may have been insufficient, and more of the bromide 

 and ammonia solution may V)e added. If the picture appear 

 too quickly, it sliows that the exposure has not been cor- 

 rectly timed. It is as well to add too little of the bromide 

 solution, as it is easy to add more if the picture be tardy in 

 developing. Under-exposure is apt to produce hard nega- 

 tives and over-exposure too little density. 



Too much attention cannot be given to the washing of 

 gelatine plates. Running water may be used — a gentle 

 stream is sufficient—or the plate may be placed face down- 

 wards (not touching the bottom of the dish) so that the 

 thiosulphate of soda may be washed out, the water being 

 clianged several times. 



.Should the negative appear to be too thin, it may 

 be intensified as follows : While wet, flow over the plate a 

 saturated solution of bichloride of mercury. Notice the 

 effect, and take care not to over-intensify. At the proper 

 moment return to stock - bottle and wash the plate 

 thoroughly ; then flow over it a solution of ammonia, one 

 ounce to eight ounces of water. This will darken the 

 image, and the plate must be again thoroughly washed. 

 The mercury and ammonia solutions need not be thrown 

 away, as they can be used till exhausted. If the mercury 

 (or intensifying) solution be found to act too vigorously, it 

 n»ay, of course, be weakened by the addition of water. 

 Simply flowing the solution once over the plate is often 

 sufficient, while in otlier cases the action of the bichloride 

 may be allowed to continue until the picture is bleached 

 white. 



While wet, a ge'atine plate must not be dried by a fire, 

 as a collodion plate may be. It has been suggested that if 

 surface-dried with blotting-paper, the plate may be dried 

 by fire-heat. If carefully done, this method is effectual. 

 When dry, the negative may be varnished in the usual way ; 

 or if only a few prints are required, the negative may be 

 printed from without varnishing, care iK-ing taken not to 

 use the paper damp. 



OX SOME PUBLICATIONS OF THE 

 CHRISTIAN KNOWLEDGE SOCIETY. 



'■pHE Society for Promoting Christian Knowledge has 

 X now for many years l>een remarkable for the singu- 

 larly able manner in which its Committee of <^leneral 

 Literature and Education have conducted the puVjiLshing 

 arrangements of the .Society. No .sectarian influences are 

 allowed to interfere with the quality and character of the 

 )x»oks published : in every case the question which the 

 committee w^ems always to have had in view is whether a 

 subject will l>e tr.-ated ably, honestly, and thoroughly. 



The Maxu.\ls of Elementahv Science, 

 for example, have been prepared, without exception, by 

 writers thoroughly conversant with their various suV)jects. 

 From Professor Fleeming Jenkin we have an admirable 

 treatise on Electricity, clearly and brightly written — and 

 we need hardly say, soundly. Mr. I.e Gros Clark has 

 written an excellent treatise on Physiology : one of the very 

 best extant, for its size. Those who take interest in 

 Crystallography will find the manual by Mr. H. P. Ourney 

 full of useful matter, verging strongly towards the mathe- 

 matical ; but the nature of the subject rendered this 

 almost unavoidable. The treatise on Matter and Motion, 

 by the late Professor J. Clerk Maxwell, is open to more 

 serious exception in this respect, having unquestionably 

 gone beyond the line which separates the profound from 

 the popular. Professor Clerk Maxwell was never able to 

 appreciate the difficulties which beginners find in dynami- 

 cal and kineiiiatical matters ; but in this little book he 

 seems to have thought that the best way to avoid them 

 was to pass boldly into the region where vectors freely 

 roam, and the result is mystery and misery to beginners. 

 It docs not content them to learn that, "as indicating 

 an operation, AB is called a Vector," and that " the opera- 

 tion is completely defined by the direction and distance of 

 the transference " ; nor is it any satisfaction to the incjuiring 

 mind to know that " the starting-point, which is called the 

 Origin of the Vector, may be anywhere." Yet this is all 

 which Professor Clerk Maxwell tells them about vectors in 

 defining these useful articles. The S.P.C.K. should have 

 a manual prepared, to which Professor Maxwell's might 

 serve as a sort of sequel ; for, unquestionably, a student of 

 matter and motion is much aided, later in his studios, by 

 the vectorial method. At present, however, it must be 

 admitted that the manual by Professor Maxwell is an 

 exception (and a solitary one) to the generally useful 

 character of these science manuals. Mr. Bernays' manual 

 on Chemistry and Mr. Newton's on Zoology are excellent 

 Of Mr. Proctor's manual on the Spectroscope we must not 

 say more than that the author has honestly endeavoured 

 to bring the work up, in clearness of exposition and 

 fulness of matter, to the standard of the series. 

 Astronomy, by Mr. W. M. Cln-istie, now Astrono- 

 mer Royal, is a useful and instructive little treati.se, 

 not absolutely free from errors (witness the sin- 

 gular inversion of the eflcct of bngitudc on time, in 

 the diagram on page 14, where places west of (ireenwich 

 have their time marked later than Greenwich time, and 

 places east of Greenwich Iiave their time marked earlier !), 

 but, in the main, sound. The illustration of orl)it8 of 

 comets at page 9.3, however, is misleading. Mr. Christie 

 explains, indeed, how and why he has changed the shapes 

 of certain cometic orbits ; but he says nothing of tlie entire 

 change he has effected in the positions of all the paths. It 

 is on such erroneous teaching that the paradoxists erect 

 their surprising structures. Jf there is one point in 

 which books of astronomy more require re-modelling than 

 any other, it is in this — the illustrations. The subject 

 which, perhaps— of all treated of by science writers— most 

 requires care in illustrating, is just the one wliich, as a 

 rule, is most villainously illustrated. 



OVKBilocsE WiBEs. — There ore 9,000 miles of telograph niid tele- 

 phone lines in the streets and on the house-tops of New York city j 

 3,500 of those belong to the Metropolitan Telephone Company. 

 Theso fipircB should be n warning to our metropolitan authorities, 

 for the number of telephone wires over liOndon is increasing at an 

 alarming rate. The Pofit-Office has spent nn enormous sum during 

 the past few years in converting the overhouso into underground 

 linos. 



