January 9, 1896J 



NATURE 



231 



gelatine or collodion must be rejected, because their tint changes 

 very quickly in the light, and they easily lose their transparency. 

 Either yellow glass must be used, or cells containing a suitable 

 liquid. 



N ellow glasses make the most convenient screens of all ; but 

 the difficulty is to find suitable glasses which are always the 

 same, and of sufficiently graduated shade. Some are excel- 

 lent, others not worth anything. Before recommending the 

 use of coloured glasses exclusively, some experiments ought 

 to be made with the help of a glass maker, in order to ascertain 

 what ought to be the exact composition of the glass so that 

 it may be reproduced with the exact tone at any time. It is 

 unnecessary to add that the glass must be homogeneous and 

 polished with quite parallel surfaces ; only glass ought to be 

 used which is coloured in mass, and not white covered with a 

 superficial layer of enamel. 



The surest method, in the case of not being able to get glass 

 of which the composition is known, would be to use liquid 

 screens, as I do. They are made with two parallel square 

 glasses cemented together on three sides by square glass rods 

 also with parallel sides, and with a thickness of six or seven 

 millimetres, and length of side seven to eight centimetres, one 

 side remaining open. If one does not wish to go to the trouble 

 (if making them, these cells can be obtained from instrument- 

 makers. Needless to say that before cementing, the glasses must 

 be carefully cleaned with a solution of carbonate of soda, then 

 with water, and lastly by being well rubbed with a piece of 

 cotton- wool dipped in alcohol ; with these precautions, no air is 

 to be feared along the sides of the cells. Before introducing 

 the liquid, care must be taken to dip the open end of the cell in 

 a bath of resin (a mixture of yellow wax and resin of equal 

 parts). For ultimately closing the cell, it suffices to fasten on 

 the edges, thus covered with resin, a little plate of glass cut to 

 a suitable size, and which must be heated on a plate of copper 

 to prevent its breaking. If found desirable the aperture may be 

 still more securely closed with sealing-wax. Thus cells are 

 obtained hermetically sealed, which can be used at every inclina- 

 tion without the liquid spilling and without air getting along the 

 joints. 



The easiest way of fixing these cells in position is to pierce a 

 circular hole in the centre of a flat piece of cork, the size of the 

 sunshade of the lens of the camera. The plate is fitted into 

 the sunshade and held by india-rubber. The screen is thus in 

 front of the lens, and it can be easily replaced by others more or 

 less dark. 



For the liquid, I have had to reject all solutions of organic 

 colours, such as aurentia, primuline, chrysoidine, for they alter 

 in the light. The simplest one is to use the bichromate of 

 potash. A saturated solution is prepared at ordinary tempera- 

 ture, to which is added, after straining, a few drops of hydro- 

 chloric acid. This saturated solution, introduced into one of the 

 previously described cells, constitutes screen (i), which should be 

 used when the clouds are very light, and the sky of a pale blue. A 

 solution of half the strength forms screen (2), which may be used 

 for well-lighted detached cirrus on a really blue sky ; lastly, screen 

 (3), consisting of one part of the saturated solution to three of 

 water, should be reserved for very luminous clouds as cumulus 

 and cumulo-nimbus. 



It is certainly more convenient and more simple to use coloured 

 glasses as screens ; but while there is a doubt as to finding suitable 

 glass, we can always be certain when using bichromate cells 

 of straightway obtaining excellent screens, always precisely the 

 same. The ones I possess have been in use two years, and no 

 precautions have been taken to preserve them. 



(2) Sensitive Plates. — Special plates must be used for yellow 

 light. The way of preparing these plates by means of ordinary 

 plates is already well known ; I did this at first. But I am 

 certain that the necessity of preparing the plates is the principal 

 obstacle which stops people taking photographs of clouds, who 

 are really desirous of doing so. However, prepared plates are 

 to be had in the trade, and they serve the purpose admirably. 



Among the types of plates called orthocromatic or iso- 

 chromatic, two have given me excellent results : Lumiere's 

 tirthochromatic plates, sensitive to yellow and green light, and 

 Fdward's orthochromatic plates. 



There is, therefore, no necessity to prepare plates, as they are 

 to be had ready-made, and, at least in most cases, quite as good 

 a-s those one could prepare personally. It has been said that the 

 sensitiveness of these plates alters very soon, so much so that 

 they are useless at the end of a few months. With regard to 



NO. 1367, VOL. 53] 



this, I can but quote the following fact. In February 1893, ^ 

 received from the firm of Lumiere, three boxes of orthochromatic 

 plates, of which the date of manufacture is unknown to me. These 

 three boxes were simply placed in a cupboard of my bureau with- 

 out any other precautions. The first box of twelve plates was 

 used in the course of 1893 '■> ^^e second, only opened at the 

 beginning of 1894, and used between the months of March and 

 November ; lastly, the third box was opened only in November 

 1894, and the two first plates gave negatives which did not differ 

 at all from those obtained from similar plates twenty months 

 before. 



Other similar boxes opened, then forgotten, for some months 

 in a cupboard, have always given me excellent results. 



I intend to continue these studies ; but it seems to me now 

 established, that if the sensitiveness of these plates diminishes 

 with time, this diminution is small enough to permit of the 

 plates being used after more than eighteen months. Under 

 these conditions nothing can be said against their use. 



Focussing is done without any difficulty on a distant object, 

 for instance on a house in bright light. If the horizon is not far 

 enough distant, an object can be taken comparatively near (at 

 least twenty-five or thirty metres), then in order that the posi- 

 tion shall correspond with infinity, move the ground-glass 



towards the lens a distance 



/ 



k 



f being the focal distance 



of the lens, and k the number of times that the distance 

 of the object which has been focussed contains the focal 

 length of the lens. For instance, if an object twenty metres 

 distant has been focussed with a lens, of which the focal 



80. 



length is twenty-five centimetres, then we get k ~ 



25 

 In order that the clouds may be in focus, the ground-glass 



must be brought a distance of -^ = 0*32 cm., about 3 milli- 

 metres, nearer the lens. Of course the focus must })e got with 

 the coloured screen, and the position thus found must be marked 

 on the base of the camera, in order that the position of the 

 frame may be known. 



(3) Dere/opment. — No motle of development must be rejected 

 a priori ; even developers called automatic, which can be bought 

 ready prepared, and which have lieen very much run down, for 

 they are by far the most convenient, and often give excellent 

 results. 



If the negative that we wish to develop contains only 

 clouds of more or less the same intensity, the automatic de- 

 velopers may be used without any risk. I have used baths of 

 hydroquinone, Lumiere's develoi^er (of paramidophenol), Sec, 

 with success. It is advantageous to use baths which have 

 already been used, and consequently containing a good propor- 

 tion of bromide ; a greater contrast is then obtained between the 

 clouds and sky, and the development can be carried further 

 without fear of fogging. 



If, on the contrary, the negative consists of clouds of very 

 unequal luminous intensity, as, for instance, delicate cirrus and 

 strongly lighted cumulus, the negative would not turn out well 

 with automatic developers containing much bromide ; the image 

 of the cumulus would appear, and be over-developed before that 

 of the cirrus had begun to show itself. In this case either a new 

 bath must be used, very diluted, without bromide, and the 

 development is then very slow, or else (which is preferable) use 

 pyrogallic acid, in employing the method recommended by 

 M. Londe. In this case the development must be commenced 

 with a very small quantity of pyrogallic acid, a little bromide, 

 and relatively enough carlxinate of soda, in such a way as to 

 make all the parts of the image appear at first, without much 

 intensity ; then the necessary intensity will be obtained little by 

 little, by the successive additions of pyrogallic acid. It is in 

 this case only, where the intensity of the clouds is very different, 

 that I think it advantageous to recommend progressive develop- 

 ment instead of pyrogallic acid. In most ordinary cases, 

 however, the automatic developers, which are more rapid, and 

 more convenient to use, act very well. 



In fact it is always as well to continue the development till 

 the image is sufficiently dense, without intensifying, which is 

 almost always possible. Negatives ought only very exceptionally 

 to be intensified : to my mind, the intensification is always 

 bad, it spoils the detail ; a renewed or feeble negative 

 is never worth as much as one that was made sufficiently dense 

 in the first instance. 



