64 Analyses of Boohs. 



prism, in obtaining at the same time, circular images of 

 the sun in red, green, and violet light, in positions agree- 

 ing with the known difference in the refrangibility of these 

 colours, perfectly free from any intermediate images. 



My method of making these experiments is, by suffering 

 the direct light of the sun to fall on a prism, and then 

 interpose such coloured glasses between the prism and the 

 eye, upon which the spectrum is received, as are found 

 best calculated to absorb the weak reflected images, and 

 leave the primary images of sufficient strength to be dis- 

 tinctly visible. The glasses which succeed best are a com- 

 bination of blue,* violet, and yellow, or rather pale orange ; 

 but as the intensity of the light of the sun is different at 

 different times, the reflected images vary in strength, and 

 different thicknesses of the coloured glasses are required 

 to absorb them ; no combination of glasses can therefore be 

 specified, which will at all times insure success. 



The greatest difficulty in making the experiment arises 

 from the want of absorbing media, which will act distinctly 

 upon the different colours ; I have, however, made the expe- 

 ment at least fifty times, and never attempted it, after the 

 first discovery, when the sun was visible for a sufficient 

 time, without succeeding. The red image is the most 

 readily formed ; next to this, the green ; but the violet, 

 although the orange glass gives such a complete command 

 of this light, is sometimes attended with greater difficulty, 

 and, to render the colour distinct, generally requires to be 

 reduced to a faint image. The three circular images are 

 separated considerably from each other, but not equally, 

 the violet being much further removed from the green, 

 than the green from the red : in the various experiments I 

 have made, I have never observed the slightest difference 

 in the arrangement I have now described. 

 (To be continued.) 



Article VIII. 



Analyses of Books. 



Outlines of Mineralogy, Geolooy, and Mineral Analysis. By 

 Thomas Thomson, M.D., F.R.S., &c. 2 vols. London, 1836. 



There is not any more important result which has emanated from 

 the discovery of the atomic theory than the demonstration that the 



• More properly, from the light they transmit, violet, and crimson. 



