444 



KNOWLEDGE & SCIENTIFIC NEWS. 



[June, 1906. 



qualities of the developer, it may be pointed out that, 

 with the exception of the phototjraphs of \'cnus, all the 

 plates recjuired to ])roduce the necessary enlargements 

 for the lunar and planetary illustrations accompanying 

 the pre\ ioas paper were developed with a solution which 

 had been several times used 18 months previously. 



In cases where it is desired to secure delicate detail 

 on a plate which has been exposed to a brilliant light, 

 such as that of the sun's disc, and also in cases where 

 the object presents strong contrasts, the plate should 

 be kept in a very weak solution until a faint image is 

 just visible, which will probably require an hour or 

 more. The strength of the developer may then be gradu- 

 ally increased, the requisite amount of density being 

 finally obtained by using a bath of normal strength. 

 The weak solution may be conveniently kept in an old- 

 fashioned collodion dipping bath fitted with a light-proof 

 cover, and afterwards transferred to the usual develop- 

 ing dish, and treated as described. It must be kept in 

 view that no amount of soaking will bring out detail on 

 a plate which has been under-exposed. The exposure 

 in all cases should be ample, and there is, consequently, 

 behind the developer, a reserve of stored up energy, 

 which can be brought into action as required. 



With a negative full of detail which is too delicate 

 for the purposes of reproduction, the contrast between 

 the ' high lights " and " shadows " may be increased 

 by taking a positive from the neg-ative by contact or by 

 the camera, a much longer exposure than usual being 

 given to a correspondingly weaker light. On the other 

 hand, where less contrast is required, the light is in- 

 creased and the exposure reduced. 



To obtain photographs of faint stars the exposure 

 necessary is so protracted that the methods employed 

 must be such as to secure impressions of the largest 

 number on a given area of the plate in the shortest 

 time, but in the case of stars and diffused nebulosities 

 showing little detail in the shape of structure nothing is 

 sacrificed by having recourse to methods which admit of 

 the exposure being largely reduced at the expense of 

 getting harder effects. For such subjects the most 

 rapid brand oi plate should, therefore, be used in con- 

 junction with a vigorous developer. Metol will be 

 found to give cleaner negatives than pyrogallic acid, 

 but if used frequently it sets up an irritating action on 

 the skin, unless the precaution is taken beforehand of 

 rubbing the fingers with vaseline. Tlie following 

 formula has been found to give good results : — 



The water should first be heated, and the ingredients 

 dissolved in the order given. The keeping qualities of 

 metol are not equal to those of glycin, and a smaller 

 quantity should be made up at a time. 



The plate should as far as possible be covered during 

 the entire process of development, which may be con- 

 tinued until the film has very appreciably darkened in 

 colour. 



If the negative after being fixed is carefully examined 

 it will frequently be found that in addition to the well- 

 defined stellar points there are numerous impressions of 

 smaller stars which are little more than visible, and are 

 too faint for printing purposes. If by intensifying the 

 negative the density of these faint images can be in- 

 creased, a resuU similar to that which would be obtained 

 by increasing the exposure is secured, and if the follow- 



ing method, which differs- only in one particular from 

 that given in photographic text-books, is employed, the 

 exposure necessary to bring out a given amount of de- 

 tail will be reduced by at least one-half, and only those 

 who have spent many a cold winter night at a guiding 

 telescope can appreciate what this means. After the 

 plate has been fixed and thoroughly washed, but before 

 being dried, it is bleached in a saturated solution of 

 bichloride of mercury in the ordinary way. It is again 

 carefully washed and placed in 10 ounces of water to 

 which one drop of strong ammonia has been added. It 

 should be allowed to lie in this solution for about an 

 hour, when two additional drops of the alkali may be 

 given, and after a shorter interval the strength of the 

 solution may be further increased if the original colour 

 of the film has not been completely restored. If 10 or 

 15 drops of ammonia are added at the outset, as is 

 usually recommended, the result will be much less satis- 

 factory, a remark which also applies to attempts to 

 intensify a negative after it has been dried. 



The accompanying photograph of a portion of the 

 Milky Way in the vicinity of Beta Cygni is reproduced 

 as an example of the amount of detail which may be 

 obtained with an exposure of two hours by developing 

 with metol, and afterwards intensifying the plate in the 

 way which has been described. It was taken with a 

 5j-inch lens of 22 inches focus, and an examination of 

 the original negative shows that the greater portion of 

 the region embraced by the photograph exhibits large 

 extensions of faint nebulous matter, broken up here 

 and there by well-marked, irregularly-shaped dark 

 channels, which appear to be associated with streams of 

 small stars. 



The method of developing and intensifying a plate 

 here referred to is, however, not applicable to strongly- 

 lighted subjects showing structural detail, and more 

 especially when the object is much brighter at one part 

 than at another. The great nebulse in Orion and 

 Andromeda may be given as noted examples. If forced 

 development is resorted to in such cases with the view 

 of securing impressions of faint stars and outlying 

 wisps of nebulous matter, the imag^e will have become 

 so dense at other parts, which have received a much 

 greater amount of light, that all detail will be effec- 

 tually obscured. Exposures made on objects of this 

 class should be developed on the lines recommended for 

 solar work, viz., by using a very weak solution at the 

 outset, allowing plenty of time, and gradually in- 

 creasing the strength of the dev'eloper as the imag'e 

 gains in density. 



By careful manipulation it is possible to reduce satis- 

 factorily the denser portions of such negatives by the 

 local application of a reducing solution as described in 

 a previous paper, but it is a matter of opinion how far 

 such treatment may be permissible when dealing with 

 objects of a nebulous character, as there is great risk 

 of introducing detail which has no objective existence. 



Another method may be employed to which less ob- 

 jection can be taken. A positive may be made from 

 the negative by contact, and after an exposure, suitable 

 for the parts which are less dense, has been given, a 

 piece of cardboard, with an aperture somewhat smaller 

 than the dense part of the film, is placed over the nega- 

 tive, which is then exposed a second time to the source 

 of light, care being taken that the cardboard is kept 

 moving during the operation, otherwise the shadow of 

 the aperture would appear on the plate. If the dense 

 part of the image increases gradually up to the centre a 

 third exposure may be given with a screen having a 

 smaller opening. From the positive thus obtained 

 another negative mav then be taken. 



