ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 721 



r in the spectrum is opaque. Thus, this particular radiation will not be 

 able to reach the slit, for the only radiations of this kind which would 

 have been able to fall upon the slit, would have had to pass through /'. 

 In order to apply these principles to the reproduction of colours the 

 author has installed the following apparatus. The single slit of a spectro- 

 scope is replaced by a series of slits very close together, viz. the fine 

 transparent lines of a web, five threads to the millimetre, as used in 

 industrial photography. This web is fixed in the opening of a photo- 

 graphic enlarger, i.e. of a box fitted at the mouth with a slit carrying a 

 sensitive plate, a convergent lens being fixed inside. A small-angled 

 prism is arranged before this lens with its edge parallel to the lines of 

 the web. The image is projected on the web ; the sensitive plate is 

 then developed and replaced. When the apparatus is illuminated with 

 white light the image is seen with its colours. Each line of the web 

 acts as the slit of a spectroscope. At the distance of distinct vision the 

 lines of the web are no longer seen and the image appears continuous. 

 The experiment was at first made with the spectrum of electric light, 

 which was reproduced complete with its colours, a positive being used. 

 With a negative the colours were replaced by their complementaries. 

 A coloured window, red and green, projected on this web was similarly 

 successfully reproduced. It is necessary that the prism should have an 

 angle so small that each spectrum should have a length less than an 

 interline, otherwise the spectra will overlap. It is also necessary that 

 the photometric plate should be re-inserted in exactly the same position. 

 Very sensitive orthochromatic plates may be used. It must be admitted 

 that the necessity of having to replace the proof in the same apparatus 

 which has produced it is not very convenient. When seen in the hand 

 the proof has the ordinary black-and-white appearance. Seen with a 

 lens it appears lined, and each line is divided into small zones which are 

 the parts of an elementary spectrum. The author suggests the following 

 as a possible improvement of the process. Insert a sensitive plate in an 

 ordinary dark camera, without a prism, but with a web of, say, five threads 

 to the millimetre. Superpose on the web a network of 500 threads 

 to the millimetre. Every luminous point projected on the plate is 

 received as a spectrum and so photographed. 



On applying the web with its network upon the developed proof the 

 colours of the original ought to be seen, the only condition being that 

 the eye should occupy the position of the objective. The system is, in 

 fact, reversible in virtue of the reasoning above given. 



L i ppm ann, M. G. — Remarques generales sur la photographie interferentielle des 

 couleurs. Comptes Bcnclus, cxliii. (1906) pp. 273-4. 



(5) Microscopical Optics and Manipulation. 



Brass, A. — Ueber die Doppelbrechung. 



Ccntralbl. Zeit.f. Opt. u.Mech., xxvii. (1906) pp. 192-4 (1 pi.). 

 Haetl, H.— Ein Modell zur Erlanteruug der Zerlegung eines linear polarisierten 

 Lichtstrahles bei der Doppelbrechung. 



Zeitschr. f. Unterricht, xix. (1906) p. 175. 

 Koebbek, F. — Ein Freihandversuch zur Ermittlung des Brechungsexponeten des 

 Glases. 



Tom. cit., p. 167. 



