A good lens should be free from scratches, striations, dull patches, due to imperfect polishing, and veins; but air bubbles do not affect its value, for it must be remembered that the shape of the hole through which the light passes does not affect the image, save only by cutting off some of the light. Thus, if a wafer be stuck to the centre of the lens, the image will be found unimpaired. Dust and dirt, however, though they do not seriously impair the definition of the image, yet cut off much light, as will occur to any one when he thinks of the difference between the light of a room, when the windows are dirty, and when they are perfectly clean. Lenses should not be left in bright sunlight, for this causes a change that slows them, the dark also injures them in certain cases, for, as all microscopists know well, darkness causes a change in Canada balsam, with which lenses are cemented together.
Mr. Dallmeyer insists that lenses should be kept dry and free from sudden changes of temperature, otherwise they may tarnish or sweat, as it is called. Any one who has been troubled with this sweating will never forget it. Our experience is that the best way to keep lenses is in small leather, velvet-lined cases. We generally keep with them a piece of soft chamois leather, or an old silk handkerchief. No compound of any kind should be used to clean lenses, if anything appears to be going wrong with them, they should at once be sent to the maker.
View-meter.
A valuable little tool is a view-meter. The handiest and compactest we have seen is that supplied in telescopic[telescopic] form.
CHAPTER III.
DARK ROOM AND APPARATUS.
Dark room.
There is no need to despair if there is no dark room, no place to build one, no means to pay for one. Some of our most successful plates were developed in a scullery, and others in the bedroom of a house-boat. |Developing rule.| In fact, the sooner the student learns to develop anywhere, the better, for no one, studying to do artistic work, should leave his plates till his return home (if he is away on a journey); they should without fail be developed the same day on which they are exposed.
Dark room.
Only for portraiture is a dark room very necessary, and you cannot do better than build one as suggested by Captain Abney, in his “Treatise on Photography,” modifying it to suit your taste and means. One thing, however, you should be careful about, and that is the ventilation, and money should not be spared on that department. |Ventilation.| The dark room can be scientifically ventilated by any good sanitary engineer. We have already, elsewhere, gone into the subject of ventilation of dark rooms, warning photographers of the pernicious effects of defective ventilation.[[11]] |Apparatus.| The best sinks are made of earthenware, as supplied by Doulton. The lamp should be large, and give a good light. |Ruby glass.| Ruby glass is, to some, injurious to the eyesight, and has been known to produce nausea and vomiting, in which cases cathedral green and yellow glass should be used. |Dishes.| The photographer will require at least eight dishes, and at the very start he should make it a rule never to use a dish save for one purpose. We consider the best dishes for all purposes are made of ebonite. They should be bought in a nest, the smallest size taking the largest plate used by the operator, and the other seven increasing in size, so that one fits into the other. This makes them more convenient for carriage. The dishes should be marked by painting on their bottoms. One will be wanted for developing, one for the alum bath, one for the changing bath, one for the hyposulphite bath, one for the acid bath in developing platinotype prints, one for the water bath in the same process, one for an intensifying bath, leaving one over for odd jobs.