248 Intelligence and Miscellaneous Articles, 



the darkening of the surface ; and hence we see that the colours must 

 cooperate in a definite ratio in order to excite the most favourable 

 impression. 



It is well known that the imperfectly pure carthamine, which occurs 

 in commerce in pink saucers, has a yellowish metallic lustre when 

 allowed to dry in a saucer, and that in time it becomes greenish at 

 the surface. Carthamine spread upon glass plates exhibits a bronze- 

 like iridescence which afterwards disappears. Dr. Stahlschmidt 

 very kindly prepared for me a series of glass plates upon which pure 

 carthamine was spread with the greatest possible evenness. On look- 

 ing through one of these, the whole plate appears of a deep red. On 

 looking at a plate, coated on one side only, by reflected light, the 

 coated side being below, the glass plate appears uniformly green. 

 But if the plate is reversed, so that the daylight, to which the back 

 of the plate is turned, is reflected by the layer of carthamine, the ob- 

 server would fancy he was looking at a plate of polished brass. If 

 carthamine is spread upon a plate of blue, yellow, red, or green glass, 

 the green colour seen by reflexion in the first case disappears, but 

 the metallic lustre seen in the second case remains unchanged. This 

 latter therefore is produced by the combination of the reflected green 

 light with the internally dispersed red light. 



The reflected green light appears with increased intensity if the 

 plate, placed in such a position that the side coated with carthamine 

 is below, is looked at through a Nicol's prism or through one of the 

 polarizing prisms above described (p. 247). This acts in the plane of 

 reflexion like a Nicol's prism perpendicularly to the principal section. 

 The cause of heightening of the colour thus occasioned is, that the 

 light polarized in the plane of reflexion, by the outer surface of the 

 glass, is got rid of. On the contrary, if the layer of carthamine is 

 uppermost, the yellowish light gradually becomes more and more 

 nearly green as the Nicol is turned round. Carthamine very slightly 

 depolarizes transmitted polarized light, and, when the light is inci- 

 dent at a very oblique angle, renders it elliptically polarized. 



I may take this opportunity of remarking that the polarizing prism 

 described by me is especially well adapted for experiments with 

 radiant heat. I have often exposed it to the concentrated heat of 

 the sun until the cork into which it was fitted began to burn, with- 

 out the prism itself being in any way injured. In using a Nicol's 

 prism in saccharimetrical experiments, the prism is often injured, if 

 a flame is brought very close to it, by the connecting layer of Canada- 

 balsam becoming blistered. The same applies to experiments on the 

 polarization of radiant heat. When employed as analyzing- appa- 

 ratus, a Nicol's prism has the advantage that the object remains 

 erect, while in mine the reflected image rotates. The latter, how- 

 ever, possesses the recommendation for a polarizing-apparatus of 

 offering a large field of view. It is preferable to Foucault's prism, 

 since in this moisture is apt to be precipitated in the separating 

 stratum and to render the surface dull, whereas in mine the surfaces 

 can at all times be easily cleaned. — PoggendorfF's Annalen, vol. exxii. 

 p. 454. 



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