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or on a levelling-stand, the developer cannot be got evenly over it without previous moistening ; and in such a case, if the preservative has contained sugar, not a mere moistening, but a careful washing will be advisable. Dark Slides.—When plates are exposed at home, the chance of accidental exposure is very slight, and the loss of a plate not very important. It is far otherwise, however, when a plate is carried many miles: the danger of acci dental exposure is greatly increased, and the loss of a plate much more annoying. When a number of dark-slides are rolled up together in a cloth, and taken in a carriage to a distance, they move somewhat, and rub against each other. Perhaps the button works round in this way, and lets the door open a little, or a shutter works by degrees, so as to let in a crack of light. Again, in drawing out a slide from the bundle, the button may catch and open, or may open the button of another slide adjoining, or a shutter may be a little withdrawn. I have found the following arrangement very convenient aud useful to avoid these troubles. To keep the shutter from accidental moving, an ingenious little catch has been con trived for me by Mr. Peace, of this city. A piece of brass, about one and a quarter inch long, and three-tenths wide, is bent across. A hole, is drilled through the short part, and notch at one end of the long part. A screw passed through this hole secures it to the end of the slide close up by the shutter. Into the thick end of the shutter a pin is driven, which goes into the notch, thus locking the shutter most securely, and removing all danger of accidental exposure by sliding or disturbance of the shutters. To prevent opening of the door, I have an extra button placed at the side. It thus becomes extremely improbable that both the buttons will be disarranged on any one slide, and with these precautions the plates may be considered as absolutely sate. The buttons placed on slides for ordinary home use are put on the door, turn under a bent wire on the edge of the frame, and have their other end turned up and rounded, in order the more easily to be taken hold of. This arrangement, convenient enough for the use for which it is intended, becomes a great nuisance if the slides are carried out. Slides for such use should have flat buttons only, and these should be placed on the frame, and not on the door. A screw at one end secures them to the frame, and they will round so as to pass over the edge of the door. Soughing the Edges of Plates.—In the collodio-bromide process the collodion is, of course, somewhat thicker than in the ordinary modes, by reason of the bromide of silver held in suspension, and, therefore, a little more care is necessary in coating. If the filing of the edges of the plate has been carelessly and roughly done with too coarse a file, it may happen that deep notches will be made, and that the plate, after coating, will show irregular bands running down an inch or more into the plate, alternately thicker and thinner. These will show, of course, in the negative, as where the film is thicker it will develop more strongly. Such plates will, of course, be failures, and the appearance of such bands as I have described may be traced with certainty to careless roughing. I fact, it may be doubted whether' any filing or roughing of the edges whatever is needed with dry plates, as these should always be edged with india-rubber benzine varnish, or have a substratum of dilute albumen. Blistering.—In all dry processes in which albumen is used in the preservative applied to collodion films, there is a tendency to blistering. This blistering depends upon osmotic action. Liquids containing certain substances in solution have an extraordinary power of forcing their way through membranes, and many kinds of artificial films are capable of acting in this respect like natural membranes. The nature of the cotton used has much to do with this blistering ; some collodions blister much worse than others, some during development, some only in the fixing and sub sequent washing. It has seemed to me that the addition of sugar to the pre servative, in the proportion of about 10 grains to the ounce, had a tendency to diminish this evil. I suppose its ten dency to be to keep the collodion film in a more porous con dition, and so to allow the liquid which has passed in to escape again. The film, when just made, and still moist, is in a very plastic condition, and has not undergone the powerful contraction which takes place in drying. If, whilst in this soft and plastic condition, its pores be filled up with a preservative containing a substance like sugar, the spon giness of the film seems to be better preserved, and, when wetted again, it returns more to the state of an ordinary wet plate than is the case with dry plates made without sugar in the preservative. In some cases this quality is very valuable ; other forms of dry plates do not need it. But 1 wish to place on record here the opinion that the action of sugar in the preservative is to keep the film in such a condition that, on wetting it, it will return to nearly the condition it had when it left the bath, and especially that by adding sugar to a preservative, and then washing it well out before applying the developer, the development will always take place with much greater rapidity than if the sugar had been omitted : not in the least that the sugar gives enhanced sensitiveness, but that its function is to fill up the pores, keep the film spongy, and, on its removal, to leave it in such condition that the developer penetrates in stantly, and acts at once on every particle that has been impressed by light. MEANS OF EQUALIZING EXPOSURE WITH WIDE-ANGLE LENSES. BY JOHN M. BLAKE.* The effects of the inequality of exposure over different parts of the sensitive plate, when using a wide-angle lens, aro well known. The gradual diminution of light, as we pass from the centre to the borders of the plate, may give, in our collodion film, all gradations from an over-exposed nega- where the light is at a maximum, to a well-exposed ambro type at the extreme limits of the field. This difficulty can be to some extent overcome by giving the best average exposure to the whole field, and shading the borders of the plate in printing ; but we thus sacrifice other important points, aud imperfectly accomplish our object. Let us now consider the various causes which tend to diminish the amount of light as we go from the centre of the field. I will mention five. First, and most important, the increasing distance of the plate from the diaphragm. Second, the oblique position of the diaphragm to any but the central portion of the plate. Third, the obliquity of beams of light towards the limits of the field, causing them to spread over more surface than their square section. Fourth, loss by reflection from the surface of the wet film before reaching the sensitive particles below. This increases in amount as the incident rays make a less angle with the collodion surface. Fifth, loss by interference of light, which may be supposed to increase in some ratio to the whole amount passing through the diaphragm, as the latter becomes in effect narrowed by its increasing obliquity. From the first cause the light will vary as the square of the cosine of the angle included between lines drawn from the diaphragm to the centre of the field, aud to any point at which we wish to compare the illumination with that at the centre. From the second and third causes the varia tion will be as the cosine of the same angle in each case. The variation from loss by reflection must be determined by experiment. The light reflected would probably exceed that from water alone, owing to the greater density of the silver solution. The loss from the fifth cause is probably of little account, except when using a very small diaphragm, or with a lens of very wide angle. * Fhiladuliihia Vnolograpker.
