PHOTOGRAPHY 471 of hyposulphite of soda. This dissolves out all the unaffected silver chloride, and leaves the picture without liability to further change. But as the tone or tint of color that it pre- sents is commonly regarded as unpleasing to the eye, it is laid in a bath containing chloride of gold, which after a while imparts to it a delicate violet hue. Toning baths, as they are termed, of various ingredients, and capable of imparting shades of a sepia and brown tint, are recommended by different operators. They are too numerous to be here described. The following formulas for the collodion process have been recommended : 1. For the collo- dion : gun cotton, 4 to 8 grains ; sulphuric ether, sp. gr. '720, 5 fluid drachms; alcohol, sp. gr. -825, 3 fluid drachms ; iodide of cad- mium, 4 to 5 grains. 2. For the nitrate bath : water, 1 fluid ounce; nitrate of silver, 30 grains; as much iodide of silver as it will dissolve. 3. For the developer : water, 1 fluid ounce; pyrogallic acid, 1 grain; acetic acid, 10 to 20 minims. 4. Or this: water, 1 fluid ounce; protosulphate of iron, 12 to 20 grains; acetic acid, 20 minims. 5. For the fixing so- lution: water, 1 fluid ounce; cyanide of po- tassium, 2 to 20 grains. 6. For the fixing solu- tion (another formula) : water, 1 fluid ounce ; hyposulphite of soda, i ounce. The follow- ing formulas may be useful in the printing pro- cess : 7. For the salting solution : chloride of ammonium, 200 grains ; water, 10 ounces. 8. For the sensitizing solution : nitrate of silver, 60 grains; water, 1 fluid ounce. 9. For the fixing solution : hyposulphite of soda, 4 ounces; water, 8 ounces. 10. For the toning solution : chloride of gold, 4 grains ; hyposulphite of so- da, 4 ounces; water, 8 ounces. If the paper previously to being sensitized has been imbued with albumen, the resulting proofs (albumen proofs, as they are termed) have a glossy and much improved appearance. It is to be under- stood that a print as well as the original collo- dion must be thoroughly washed in clear water after the process for fixing or toning has been completed ; otherwise it will be liable to a spontaneous fading away. Among the appli- cations of photography must not be omitted the interesting one of the stereoscope. Stere- oscopic photographs may either be made by a purposely constructed camera with a pair of lenses, or by a single camera set successively in two different determinate positions. The illusion of the stereoscope gathers force from the truth of the photograph, and such pictures, from the air of solidity that they present, give a very striking result, not only in the case of portraits from the life, but also in landscapes, and especially in architectural objects. Though photographs as now produced by the best ar- tists are of very great beauty, they are never- theless very imperfect. They do not critically represent the exact order of light and shade ; and what is a still greater defect, they do not represent the order of luminosity, as dependent upon the coloration of the object. To the eye the yellow is the brightest color, the intensity of the light declining as we go to the violet end of the spectrum on one side, and to the red on the other. But in all the silver preparations in use among photographers, the indigo ray produces the greatest effect, and therefore may be said to possess the greatest illuminating power, and from it the intensity declines to- ward the violet on one side, and ceases on the other before the yellow is reached. The effect of this in a photograph may be easily under- stood. If two pieces of paper, one painted light yellow and the other deep indigo, be ex- amined by the eye, the former impresses us most vigorously, and we speak of it as being bright in comparison with the other. But if a photograph of these two pieces of paper be taken, the deep indigo will come out white, and the light yellow completely black. So the real order of their visual intensity is reversed in their photographic representation. For this reason photography has given unsatisfactory results in its application to landscapes. Noth- ing would tend so quickly to the improve- ment of photography as the invention of some means for the accurate admeasurement of light ; i. e., some instrument that would answer for the luminous agent, as the thermometer answers for heat. The contrivances that have been recommended may answer well enough in the hands of an accomplished chemist, but are un- suitable for the common operator. Among them may be mentioned the galvano-photome- ter of M. Becquerel, the chlorine and hydrogen photometer of Dr. Draper, the photometer of Bunsen, and the test paper of Roscoe. Owing to the want of durability of photographs ob- tained by the aid of salts of silver, attempts have been made to substitute for those com- pounds others not liable to change. Among such may be classed carbon, which is alto- gether unalterable in the air. The carbon pro- cess has now reached a degree of perfection that will bring it into competition with the older methods. Improvements have also been made in the operation of development; that known as the alkaline development gives very satisfactory results. Recently it has been an- nounced by Prof. Vogel that the sensitiveness of dry plates is increased by adding to the collo- dion certain coloring matters, such as fuchsine and chlorophyl, which impart a sensitiveness to the green and yellow rays. Promising at- tempts have been made by Col. Stuart Wortley, Mr. Carey Lea, and others, to get rid of the use of the nitrate bath, by covering the plates with an emulsion containing a salt of silver instead of with iodized collodion. This is the emulsion or collodio-bromide process. From what has been said respecting visual impres- sions and photographic representations, it will be perceived that the agent which accomplishes the latter is not light, and that therefore the term photography is in truth a misnomer. Among those chemists who have examined the scientific connections of this subject, clif-
