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SUMMARY OF CURKENT RESEARCHES RELATING TO 



indices can be determined within these limits to two limits in the third 

 place of decimals, if a sodium flame be the source of illumination. The 



Fig. 34. 



Fig. 35. 



instrument is calibrated by means of this light, and a table of indices 

 corresponding to every division of the graduated scale within the effec- 

 tive range is supplied with the instrument. Made by Swift and Son. 



(4) Photomicrography. 



Photography in Natural Colours. — In the Journal for December 

 1006, p. 720, an abstract was given of G. Lippmaim's new method of 

 photography in natural colours. From correspondence which appeared 

 in " Nature," * we note the curious coincidence that the method has 

 been independently invented four times, it having been twice' pre- 

 viously invented in this country and once in France. As the methods 

 diflPer in certain details and suggestions, we give short abstracts. 



F. W. Lanchestert takes a grating consisting of a number of opaque 

 parallel bars, with spaces between less than the width of the l)ars, and 

 places it between the camera and the object, and as near to the latter as 

 possible, so that both are practically in focus on the photographic plate 

 at the same time. The camera has a prism arranged in front or Ijehind 

 the lens, with its axis parallel to the bars of the grating, the dispersion 

 being such that when the camera is focused on the grating the images 

 of the slots form a series of spectra on the focusing screen or plate. 



The bars of the grating are sufficiently numerous to prevent the 

 picture being unduly broken or disjointed. In taking a photograph, an 

 isochromatic plate is used ; the resulting negative contains a record of 

 the colours of the object in the form of shaded lines of varying in- 

 tensity. A positive is then made on a lantern plate ; this is placed in a 

 similar or in the identical camera with which the photograph was taken, 

 and in the position originally occupied by the photographic plate. The 

 photograph now appears in natural colours, as soon as the grating is 



* Oct. 4 and Nov. 29, 1906. 



t English Patent Specification, No. 16548, 1895. 



