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Mat 22, 1885.] THE PHOTOGRAPHIC NEWS. 331 present themselves annually for examination in this subject con tinues to increase, showing that it is no mere whim of those who seek a superficial acquaintance with every branch of science, but a desire by those engaged in the practice of photography to obtain that technical training which the Council have placed within the reach of all. The examination consists of two grades, Ordinary and Honours, certificates (first and second class) being awarded in each grade. Prizes.—Honours:—1st prize, £5 and a silver medal; 2nd prize, £2 and a bronze medal. Ordinary :—1st prize, £3 and a silver medal; 2nd prize, £3 and a bronze medal; 3rd prize, £2 and a bronze medal; 4th prize, £1 and a bronze medal; Sth prize, bronze' medal. The following are the questions given by the examiner, Captain Abney, B.E., F.R.S. The results will be made known in August next, Instructions. The Candidate must confine himself to one grade only, the Ordinary or Honours, and must state at the top of his paper of answers which grade he has selected. He must not answer questions in more than one grade. If he has already passed in this subject in the first-class of the Ordinary grade, he must select his questions from those of the Honours grade. The number of the question must be placed before the answer in the worked paper. Three hours allowed Jor this paper. Ordinary Grade. (Not more than eight qtiestions to be answered.J 1. Give an account of the Daguerreotype process. 2. A print on sensitized albumenized paper on being taken from the printing-frame is immersed in ordinary spring-water, which thereon becomes milky. What is the cause of the milki ness in the water ? Put your answer into chemical symbols, if you can. 3. When choosing a camera (1) for the studio, and (2) for landscape work, what special characteristics should guide you in your choice ? 4. What is a “view-meter? ” Describe any one with which you may be acquainted. 5. During development, how can you distinguish between a properly-exposed, an under-exposed, and an over-exposed gela tine plate ? 6. In a photographic print from a cloud negative, the sun appears as a black instead of as a white disc, all the rest of the print being in proper gradation. What is the cause of this peculiarity in the appearance of the sun ? 7. Wishing to convert half-a-sovereign into chloride of gold (Au Cla), how should I proceed ? If the gold in the half- sovereign weighs 60 grains, how many grains of chloride of gold should I obtain ? Au=197, Cl=35-5. 8. Why are the apertures in the stops of lenses round and not square ? Illustrate your answer by a diagram. 9. On what phenomenon does the possibility of producing a powder picture depend ? Give an outline of any powder pro cess you may know. 10. Give a detailed account of the method you would adopt to make a lantern slide from a whole-plate negative. Honours. (Not more than eight questions to be answered.) 1. A certain light consists mainly of blue-green rays. Its spectrum has to be photographed. What compound of silver would be the best to use on the photographic plate, and why ? 2. Write an account of a process for printing by development, commencing with the preparation of the paper to the mounting of the print. 3. It has been said that a gelatino-bromide plate, in compari son with a wet plate, is more sensitive in winter than in summer. On what grounds has this statement been made, and give your opinion as to the soundness of the argument ? 4. Discuss the question of “ depth of focus ” in a lens. 5. Give an outline of a method of producing from a photo graph, in which there are half-tones, a printing surface to be set up with type, and discuss the merits and demerits of such a process. 6. What is the action of the addition of a small quantity of sodium hyposulphite to a ferrous-oxalate, or ferrous-citrate developer ? You should state your own idea as to the chemical reaction that takes place. 7. Wishing to make a star map, by photography, of some small portion of the heavens, how should you proceed to carry out your idea ? Give some details of the apparatus you would employ. 8. Give your own opinion as to the best form of sensitometer as regards construction, range, and graduation. You are not required to describe any particular form which can be obtained commercially, but rather to discuss the subject. 9. A platinotype print is said to be “ developed,” as is also a wet plate. In what respect does the development differ in the two cases ? 10. A negative is intensified with mercuric chloride, followed by ammonia ; another with mercuric chloride and silver cyanide ; another by ferrous sulphate and silver nitrate. Explain chemi cally what takes place in the three instances. AN ELECTRICAL STANDARD FOR MEASURING LIGHT.* Our engraving represents a new form of arranging an incan descent electric lamp with reference to its use as a standard light for photographic purposes, and is the outcome of a long series of experiments by Mr. Thomas A. Edison and his assistant, Mr. John Ott, in charge of Mr. Edison’s laboratory. The strength of the light required was to be equal to one candle power. When the matter was first introduced to Mr. Edison, he was of opinion there would be no difficulty in obtaining a means of accurately measuring and controlling the resistance of such a small lamp if a battery was employed. The original plan was to interpose a known resistance in the main circuit with the lamp, which could be varied, and also an amperemeter or a voltmeter for measuring the variations of the current ; but, after a large number of experiments, it was found impossible to make an instrument delicate enough to accurately measure the very low resistance in the lamp, which is said to be equal to about three-fourths of an ampere. Mr. Edison then turned his attention to the utilization of the electrical compensation balance invented a few years ago by Prof. Poggendorf, which is generally recognised as being the most delicate method of measuring electro-motive force of batteries, and at the same time has the advantage of being entirely free from any detrimental polarization. In this method ef measurement the currents from two batteries are so balanced by the inseition of a variable resistance that, if a galvanometer is inserted in the circuit, no traces of a current can be perceived. The arrangement as shown consists of a standard constant battery, a galvanometer, a key, a rheostat or resistance wire made in two sections, two parallel brass rods arranged directly above each section of the wire, provided with adjustable collars, which connect the bars to the sections of wire and a switch, all fixed upon a base which rests upon a photometric testing box. Within the latter, supported upon a sliding board, is the standard electric lamp. Hinged to this board is a long wood rod, which, when the side of the box is closed, as it is intended to be for actual work, permits the operator to move the lamp at the open end to different dis tances from the sensitive plate, held in a plate-holder slide, shown at the opposite end. The lamp is connected by flexible cords to the binding posts leading to the main battery, and one cf tho section of the rheostat wire. The apparatus is intended to be used in the photographic dark room. The cell of the standard battery, S, is the standard by which the electro-motive force of the Fuller, or main lamp, battery, M, is measured. The battery, S, which is comparatively new, was devised by Mr. George Wirt, who is connected with the Western Electric Mfg. Co., of New York, and is a modification of the well-known Daniell battery. It is so constructed that the fluids cannot become disturbed or mixed through any slight jarring. It con sists of three square bottles, 1} inches square by 4} inches high, with a neck 2 of an inch in diameter by 1 inch long, securely clamped together with metal screw rods at the top and bottom, and held in an upright position by a light wood framework, as shown in the engraving. In the upper part of the adjoining sides of bottles I. and II. is drilled a small hole 8 of an inch in diameter, and in the lower part of the adjoining sides of bottles II. and HI. are similar holes, all arranged to correspond with each other. • Communicated to the Editor of the Scientific American,