TRANSACTIONS OF THE SECTIONS. 9 



only greatly to extend the variety of experiments and illustrations, but 

 much to improve the splendour of them all. The most important of 

 these is in the analysing part of the apparatus. Having obtained an 

 unusually fine plate of tourmaline, he tried various experiments with it 

 and other means, from which it appears much more can be done with a 

 bundle of films of mica, when bleached and properly constructed for 

 analysing, than can be effected by any tourmalines, however good. 

 Prof. Forbes, in his experiments on the polarization of heat, first em- 

 ployed bundles of bleached mica, — his process of bleaching which, by 

 heat, he has already published ; but, for experiments on light, the pro- 

 cess must not be carried so far, and requires to be conducted with great 

 care, for when raised beyond a bright red heat, the mica blisters and 

 becomes unfit for these purposes, being broken into very small portions, 

 that are incapable of transmitting a clear and distinct image, from the 

 number of unequal refractions which the light undergoes in different 

 parts : whilst the heat, if properly regulated, will drive off all the colour 

 of the mica, without its being blistered; it may afterwards be easily 

 divided into sufficiently thin laminae, so that about eighteen of them, 

 placed between two plates of thin glass (to protect them from being 

 scratched), form the best means of analysing, and allowing both 

 complementary images may be shown at the same time, — by using 

 two screens, one to receive the refracted image, and the other to receive 

 the reflected image, — thus furnishing the means of exhibiting, with 

 singular effect, all the beautiful phsenomena of polarized light. 



A Letter to the Rev. William Whewell, President of the Section on th^ 

 Chemical Action of the Solar Rays. By Sir J. F. W. Herschel 

 Bart., F.R.S. ' 



Slough, Aug. 28, 1839. 

 My dear Sir, — May I take the liberty of requesting that you will 

 mention to the Physical Section of the British Association a very re- 

 markable property of the extreme red rays of the prismatic spectrum, 

 which I have been led to notice in the prosecution of my inquiries into 

 the action of the spectrum on paper, rendered sensitive to the chemical 

 rays by Mr. Talbot's process, or by others of my own devising. 



The property in question is this, — That the extreme red rays, (such I 

 mean as are insulated from the red of the spectrum by a dark blue 

 glass coloured by cobalt, and which are not seen in the spectrum un- 

 less the eye be defended by such a glass from the glare of the other 

 colours,) not only have no tendency to darken the prepared paper, but 

 actually exert a contrary influence, and preserve the whiteness of paper 

 on which they are received when exposed at the same time to the action 

 of a dispersed light sufficient of itself to produce a considerable im- 

 pression. I have long suspected this to be the case, from phsenomena 

 observed in taking photographic copies of engravings ; but having at 

 length obtained demonstrative evidence of the fact, I think this may 

 not be an improper opportunity to announce it to the President of the 

 Physical Section of the British Association. 



