ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 223 



illuminated by a nniforin source of white light. Reconstruction of the 

 colour picture can also be effected by placing a lamp and condenser at 

 the back of the lantern slide and projecting the picture on to a screen. 

 In landscape photography, when the grating cannot be conveniently 

 arranged near the objects being photographed, the picture is first pro- 

 jected on to the grating, by an additional objective. A field lens is then 

 required between the two objectives. When taking the photograph a 

 light filter is used to cut off the ultra-violet rays. 



Julius Rheinberg * suggests the arrangement as above, in which the 

 picture is first thrown on a grating of 300 lines per inch for example, 

 with bars double the width of the spaces, the picture and grating being 

 projected by the second objective, with a prism behind it, on to the 

 photographic plate. It is pointed out that the method necessitates but 

 a single negative, a single exposure, and no colour screens except an 

 orthochromatic filter when taking the photograph. It might be looked 

 upon as an extension of the Joly process of colour photography, the 

 difference being that the artificial lines in three colours are replaced by 

 real spectra produced optically. Regarding practical points, the grating 

 used would require to have the dark interspaces perfectly opaque. The 

 orthochromatic filter used would require to be such that the spectrum 

 gave a deposit on the plate equal in density throughout its entire length. 

 It would differ, therefore, from the ordinary orthochromatic filter, 

 designed to fulfil the condition that the deposit on the plate corresponds 

 to Maxwell's colour curve of visual luminosity. To view the photo- 

 graph, a finely ground glass screen, or other light-diffusing medium, 

 would have to be placed in contact with the positive. It is suggested 

 that if, at a future date, paper ruled or printed with imitation spectra 

 as fine as 150 per inch could be made, photographs could be produced 

 in natural colours by this method, as easily and quickly as ordinary 

 photographs at the present time. 



Andre Cheron f describes his method, which is essentially similar to 

 that described in the two foregoing abstracts, but he makes provision 

 also for the use of a stereoscopic camera instead of an ordinary one. It 

 is pointed out that as the positives are placed in the original camera for 

 viewing purposes, it is a simple matter to place an ordinary stereoscope 

 behind the pictures and see them in relief, the only further adjunct 

 necessary being to have some large field lenses in contact with the 

 positives. It is suggested that for stereoscopic work the lines of the 

 grating should run horizontally in the one-lens system, and vertically 

 in the other. To obtain exact colour registration when viewing the 

 positives, the gratings can be moved by a micrometer screw. 



Purifying Gelatin. | — The following useful method is given in 

 "Die Photographische Welt" for purifying gelatin for photographic 

 purposes, and should be particularly useful for such as is to be used 

 for making colour filters. The gelatin should be broken up into small 

 pieces and soaked in water, which should be changed every half hour, 



* British Journal of Photography, Jan. 1904, p. 7. 

 t French Patent Specification, March 1906. 

 J English Mechanic, Ixxxiv., (1907) p. 627. 



