1878.] to the B( product ion of Maps and Plans. 69 



on the front surface and should be hirge enough not to cut off tlie oblique 

 rays entering the lens. A large solid reversing prism is expensive and heavy, 

 but for moderate sizes an efficient instrument may be made by building up 

 a hollow prism with glass plates and filling it with a trans])arent fluid having 

 a suitable index of refraction, M. Derogy, of Paris, has just invented an 

 ingenious and economical method of employing a reversing prism by 

 placing a small prism between the lenses. I have not seen any results of 

 this arrangement, but it seems likely to be as effective as it is simple. 



(4.) By what is known as the ' dusting on' or ' powder' process. A 

 glass plate is coated with a mixture of gum, sugar and bichromate of potash 

 dissolved in water, thoroughly dried with heat, and then exposed to light 

 under a negative. After removal from the printing frame, the gummy 

 film is dusted over with very fine plumbago which adheres to it in 

 inverse proportion to the action of light, i. e., those parts on which the light 

 has acted refuse the powder in proportion to the intensity of the action of 

 light, while the protected parts, attracting moisture from the air and so 

 becoming ' tacky,' take the powder readily, and thus an exact transcript 

 of the original negative is produced, but reversed. This method is simple 

 and effective and seems to be one of the best that can be employed when 

 a mirror or prism is not available, or when, as is frequently the case, 

 the reversed negative can only be obtained by coj^ying from a single 

 original unreversed negative. 



(5.) By making a copy, either in the camera or by contact, on a film 

 of collodio-bromide of silver. The image is developed as usual by the 

 alkaline method, and then treated with nitric acid which dissolves the 

 reduced silver in the exposed parts of the film, leaving the bromide in the 

 unexposed parts ; the plate is then again exposed to light and developed. 

 This method, proposed, I believe, by the late Mr. Sutton, is said to give 

 very good results, but the use of nitric acid is an obvious disadvantage. 



(6.) By copying in the camera, first making a transmitted positive 

 either by the collodion process or with a special pigmented gelatine tissue. 

 This method is most useful when the reversed negative is required to be 

 either larger or smaller than the original. 



(7.) By means of the reversing action of the red and blue rays of the 

 spectrum. This method is a discovery of my own and has not yet been 

 thoroughly worked out ; it is, however, simple and could, I believe, be suc- 

 cessfully utilised. A film of collodio-bromide of silver stained with anilin 

 blue is exposed to light for a few moments, then placed under a negative in a 

 printing frame in front of which is a sheet of red glass, and exposed to light. 

 The action of the light passing through the red glass in the clear part of 

 the negative is to neutralise or destroy the effect of the previous exposure 

 of the plate to light, and on development a more or less perfect reversed 

 10 



