720 SUMMARY OF CURRENT RESEARCHES RELATING TO 



scope, invented and described as far back as 1901.* In this instrument 

 a strong beam of light is axially directed through a test-tube filled with 

 the fluid to be examined, the tube being in a dark box. The light- 

 trajectory is observed perpendicularly through a lateral ocular-tube. If 

 the fluid is capable of fluorescence, or in a stricter sense not optically 

 empty, the appearance presented is that of a luminous (fluorescent or 

 opalescent) beam. A polarising prism applied in the ocular opening 

 distinguishes between fluorescence or opalescence, for the latter, being 

 polarised, is extinguished by rotation of the prism. Tswett's lumino- 

 scope does not render possible the direct observation of ultramicroscopic 

 particles, but it establishes in a general way the presence of such 

 particles. In physiological-chemical investigations (e. g., of chlorophyll 

 pigments), where the observer has to be specially careful with regard 

 to purity or accuracy of the solutions, or where he has to deal with mere 

 traces of fluorescent materials, he will find the luminoscope an indis- 

 pensable instrument. 



(4) Photomicrography. 



Principles on which Direct Photography of Colours Depends ; 

 Direct Colour-Photography Depending on Prismatic Dispersion, f 

 M. G. Lippmann points out that two conditions are necessary in order 

 that a photographic proof may reproduce the colours of the model. 



1. The sensitive plate should clearly distinguish the differences 

 existing between the various radiations combined in an incident ray : in 

 other words, the system employed should analyse the incident ray. 



2. In order that this incident ray, after impact, should be recon- 

 stituted with its colour, the system employed should be reversible, so as 

 to effect the synthesis of elementary colours. Now, prismatic dispersion 

 as used in a spectrocope is such a system. A photographic spectroscope 

 is composed essentially of a slit /, a prism, a lens, and a sensitive plate. 

 It evidently effects the analysis of the light incident on the slit ; thus it 

 remains to show that the apparatus is by itself reversible and that it does 

 reconstitute after impact, the coloured light which has impinged on the slit. 

 Let us suppose that the sensitive plate has been developed, a positive ob- 

 tained, and the plate replaced by the positive. If the slit has been illumin- 

 ated by (e.g.) red rays, these rays will have produced in the spectrum an 

 image r of the slit. This image is transparent on the positive proof and 

 constitutes a kind of slit which, when the plate has been replaced, is 

 the conjugate image of/. Inversely / is the conjugate image of r. It 

 follows that, if /be illuminated with white light, the transparent region 

 r will receive only the rays which have formed it, and will only transmit 

 those. If the light is inverted and r is illuminated by white light, only 

 those rays will be transmitted which have left their trace on r. This 

 reasoning applies to light of any refrangibility whatever. When the 

 positive proof is replaced by its negative the slit is illuminated by a light 

 exactly complementary to that radiation which had acted on it, and thus 



* Vorrichtung zur Beobachtung von Fluoreszenz- und Opaleszenzerscheinungen, 

 Zeit. Physik. Chemie, xxxvi. p. 450 ; Constitution physicochiinique du Grain de 

 Chlorophylle, Trav. de la Soc. des Naturalistes de Kazan, xxxv. p. 58. 



t Comptea Rendus, cxliii. (1906) pp. 270-2. 



