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Jone 4, 1869.] THE PHOTOGRAPHIC NEWS. which it reaches the photographer, it is a mass of delicat® crystals. If pinholes show themselves with tannin, the pro portion used must be diminished. It does not, by any means, follow, that because one operator uses a 20-grain tannin solution without being troubled with pinholes, that another will have the same immunity. A great deal will depend upon the pyroxyline. The film and the crystalline substance represent two opposing forces. The latter strives to crystallize, which is opposed by the colloid character of the film. Which will obtain the mastery will depend, not only upon the quantity of the crystalline substance, but upon the cotton, and the quantity of it present. A thin collodion will leave much less pyroxyline on the plate, and the tendency to crystallize will be proportionately greater. 3. Another mode of avoiding pinholes will be to avoid crystalline substances as preservatives. Coffee, for example, and albumen, do not tend io crystallize, but dry up to what is technically called a varnish ; that is, a colloid mass per fectly homogeneous. Such preservatives cannot produce pinholes; or, if pinholes show themselves with these pre servatives, they must depend upon some sort or other of bad manipulation, most probably dust settling on them during drying. I have before spoken of the favourable action of sugar in keeping open the pores of dry plates; its tendency to pre vent pinholes must also be classed amongst its good qualities; both of these are shared by gum, a substance which is destined, for many reasons, to be of the greatest value to the worker in dry plates. Wet Plates. The subject of pinholes in wet plates has been so much better studied than the foregoing, that there is but little that need be said on the subject. Want of filtration is a not uncommon cause; the motes floating in the bath rest upon the film, shelter it from the light, are removed in washing, and the unexposed iodide and bromide of silver underneath are dissolved out in fixing, and leave a trans parent spot in the lights or the half tones. I do not know whether it has been before remarked in print, although every photographer must have observed, how much more apt these transparent spots are to come in the sky than else where. The reason is not difficult to find. The part of the plate that receives the foreground is that which comes first out of the bath anil remains uppermost in the draining, so that specks and motes naturally are carried by the descend ing fluid into the lower part of the plate, which is to receive the image of the sky. Just in the same way operates the iodo-nitrate of silver, except that these crystals may not only be deposited on, but may form in, the film. In this case, the preference which these crystals also show for the sky admits of a different explanation. The bath solution continues to settle down the plate after draining, and whilst the plate is in the slide. As evapora tion goes on simultaneously, the solution on the lower part of the plate will always be more strongly charged with silver salts, and, if the delay is long, the difference may be considerable. Of course, the opportunity for the formation of the iodo-nitrate crystals increases with the concentration of the fluid. ON EXPOSURE. BY 0. B. OREBN.* In the absence of a paper on exposure by one who has had much practical experience of it in connection with the collo- dio-bromide process, I have penned a few observations which may interest our members, and, perhaps, draw the attention of some to a branch of our art-science to which little attention has been given by our Association. In the first place, I propose to consider what we mean by exposure ; next, what is the effect produced by the light ; * Read at a meeting of the Liverpool Amateur Photographic Association, May 25, 1809. • and then consider the reasons there are for believing that exposure may be almost indefinitely shortened. What we mean by exposure is, allowing the actinism of light to have access to the chemicals in the prepared film ; and this leads us to inquire, What is actinism, which pro duces the wonderful changes with the effects of which we are so familiar ? How is the change produced, and what is the nature of the change? Actinism is a term generally used to express that peculiar property the rays of light have for producing chemical changes ; but I would give it a wider meaning, and consider it to be the property which light has of producing physical as well as chemical changes. In the prepared film wo have certain chemical compounds consisting of atoms or molecules, held together by their in herent attractive force. After the film has been exposed to light for the usual time in the camera, no chemical change can be detected to have taken place in the chemicals ; all the silver is still found to be in combination with the other elements as before exposure. What, then, is the nature of the change, if it be not chemical ? I am disposed to think the change in the film consists of a kind of polarization of the molecular elements forming the chemicals. I think this change is brought about by the impact of the actinic waves of light, which produce the power called repulsion, the effect being to cause polarization and consequent vibra tion of the molecules, which weakens, but does not liberate, the attractive force existing among the atoms of the chemi cals. The molecules of the chemicals in the developer are then enabled, by their inherent attractive force, to form new compounds, which, otherwise, they would not be able to effect. These ideas will be better understood if I condense, from Winslow’s admirable work on “ Force and Nature,” the effect of a blow of a hammer upon a rod of iron or hard steel held in the line of the magnetic dip. The impact communicates the force existing in the falling body to the body struck, which, propagating itself from molecule to molecule, is counteracted in turn by the repulsive force of the same molecules. Thus vibrations—that is, molecular motions—ensue, and the iron becomes magnetized, one end being positive, the other negative. If the iron be allowed to rest, it gradually loses its magnetic power, and returns to its normal condition. 1 do not say that the change which takes place in the chemicals is magnetic; but I think the same force—that is, repulsive force—which, acting in the iron, produces the effect called magnetism, may be generated in the chemicals by actinism, and cause vibration of the molecules, and so counteract or lessen, to some extent, the attractive force by which they exist. It is a law in nature for all things to return to their nor mal condition after being disturbed, and, consequently, we find, after exposure, the film gradually loses the latent image; or, in other words, the molecules of the chemicals lose their newly-acquired power, and, like the molecules of the iron, slowly, but certainly, return to their normal state. Some chemicals may, and doubtless do, retain the effect of actinism longer than others, and some preservatives, by their presence, assist the chemicals in retaining the effects of light longer than others. The reasons why these chemicals possess these varying properties possess much interest, and deserve the closest investigation. The certain tendency which the exposed chemicals have to return to their original condition explains the reason there is for prolonging the exposure of the plates if we wish them to retain the latent picture for a long time before being developed, and also the necessity of develop ing immediately if we desire to reduce the exposure to a minimum ; and I may add that, if at any time you should make a tour with dry plates, and returning home find, on developing one or two, that they are much over-exposed, if you keep the others a few weeks before applying the developer you will get better results than you otherwise would.
