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be nearly 9 inches, and of b' nearly 9-4 from where the axis cut the lens, the place of no distortion, as before, being 7-56 inches. In this case the images are more proportion ally displaced the further they are from the axis. If we take an equi-convex lens of the same focus, the respective distances of a' and b' on the screen would be nearly 5-4 and 4-4 inches respectively, from axis of the lens, again showing that the distortion is barrel-shaped, and more pronounced than in the plano-convex lens. From these two examples it will be gathered that the form of lens materially alters the amount of distortion, and points to the employment of a meniscus form. Now let us see the effect of thickness of a lens of double curvature on the distortion. Ilg. 38. Let EHGF be a double convex lens, and let AB be a ray refracted to C in the lens, and emerging along CD. In a theoretical lens the point E would be in contact with B, and having the same curvature as it has at that point, the ray would be slightly differently placed, and at the same time the ray is lineally displaced by the difference between CC'and EE', viz., CK. This shows that the thickness of a lens makes some difference in the distortion that arises from the curvatures. The following diagram shows graphically the displacements* that take place in the case of the plano-convex lens which we have taken as an ex ample. The rays are all supposed to strike the convex surface at the same angle of 30°. Fig. 39; If a lens be used without a diaphragm for oblique rays, the astigmatism is such that the focus is confused ; but, at the same time, the rays, whose direction in passing through the lens also pass through the optical centre, will form part of that confused image. If such rays could be picked out, there would be no distortion, except a small amount, which would be due to the thickness of the lens. Such could be used—and are—when the diaphragm is in contact with the lens ; but unfortunately, in such a posi- * There would be also a slight displacement (caused by the thickness) of the ray passing through the optical centre. tion, the curvature of the field is such that no focus on a plane surface can be obtained unless the aperture in the diaphragm be very small indeed. GLASGOW AND WEST OF SCOTLAND AMATEUR PHOTOGRAPHIC ASSOCIATION’S EXHIBITION. The Annual Exhibition of work done entirely by members of the Association will be held about the middle of November, when the following prizes will be competed for :— Section A.—Limited to Prints from Negatives taken since the formation of the Association in January, 1883. 1. Best Land scape, size of print, 7 J by 41, and under, £2 2s. ; 2. Second Best Landscape, size of print, 74 by 4}, and under, £1 Is.; 3. Best Landscape, size of print, above 74 by 4}, £2 2s.; 4. Second Best Landscape, size of print, above 74 by 4), £1 Is.; 5. Best Figure Study, size of print, 74 by 4}, and under, £2 2s.; 6. Second Best Figure Study, size of print, 74 by 4}, and under, £1 Is.; 7. Best Figure Study, size of print, above 74 by 4}, £2 2s. ; 8. Second Best Figure Study, size of print, above 74 by 4}, £1 Is. ; 9. Best Instantaneous Picture of Moving Objects, size of print, 7+ by 4}, and under, 15s.; 10. Best Instantaneous Picture of Moving Objects, size of print, above 74 by 4}, 15s. Section B.—For Prints from Negatives of any size, taken at any time. 11. Best Landscape, £2 2s.; 12. Second Best Land scape, £1 Is. Certificates of Merit will be awarded for the Best Transparency, Enlargement, and Portrait taken in the Open Air. Conditions. 1. The prizes will be awarded by vote of members. A voting paper will be given to each member before the Exhibition, to be filled up according to the instructions on the voting-paper. 2. The money value of the various prizes is given opposite, but successful competitors must select some piece of apparatus, &c., of the prize value, upon which an inscription may be put. ' 3. Each exhibit must be suitably framed, and bear the name of subject, class for competition, and member’s name on the front. 4. Not more than four pictures for competition in each class to be exhibited by any member; but there is no limit to the number of pictures not for competition sent in by any member. Such pictures must be marked distinctly “ Not for competition.’’ 5. No picture that has been previously shown at a public exhibition will be admitted for competition. 6. The Exhibition will be open for two days, from 10 a.m. till 10 p.m., and the admission will be by ticket only. Tickets for members and friends may be had free of charge at the October Ordinary Meeting, or on written application to the Secretary. 7. The term “ Landscape ” includes seascapes, tree studies, river views, waterfalls, ruins, &c. A “ Figure Study ” may be defined as a picture containing at least one human figure, or group of figures, with accessories, the essential points being, firstly, that the whole composition shall suggest some definite idea, or tell its own story. Secondly, that the figures form the most prominent part of the picture. Portraits pure and simple are not included under this head. 8. Members joining the Association in October, 1883, will be permitted to compete; but the proposal forms, duly filled up, must be sent to the Secretary at least a fortnight before the second Tuesday in October (Rule X.). EorxeSgonDence. PHOTOGRAPHIC COPYRIGHT. Sir,—Enclosed I beg to hand you form of instructions for registration of photographs, &c., which can be obtained here, and which you may think of sufficient interest to your readers to notice in one of your issues.—Yours obediently, Chas. Robert Rivington, Stationers' Hall, E.C., October 12th. Registrar. Copyright.—The author, being a British subject or resident within Her Majesty’s dominions, of any original painting, draw ing, or photograph (not sold before 29th July, 1862), has the exclusive right of copying, engraving, reproducing, and multi plying such painting or drawing, and the design thereof, or such photograph, and the negative thereof, by any means or size, whether made in the Queen’s dominions or not, for bis life and