21 



the yellow; but that, from the full deep green, the illuminating 

 power decreases very sensibly, the blue being nearly upon a par with 

 the red, while the violet is still more, and the purple the most defi- 

 cient of any. 



From a collective view and a due contemplation of these well 

 established facts, it is inferred, that perhaps they may lead to the 

 discovery of different chemical properties in the different coloured 

 rays ; and that the various degrees of heat in different coloured 

 flames will probably be fully explained on the principles that may be 

 deduced from them. 



Here follows a section in which the author advances a position, 

 that radiant heat is of different refrangibility, and that it is subject 

 to the laws of the dispersion arising from this different refrangibility. 

 We perceive from the drift of his arguments here advanced, that he 

 inclines in favour of a distinction between calorific rays and colorific 

 light, both being liable to refraction, but under different angles, and 

 hence probably possessed of different momenta. This, if admitted, 

 will apply to the invisible heat of red-hot iron gradually cooled till 

 it ceases to shine, and likewise affords a solution of the reflection of 

 invisible heat by concave mirrors. 



We come next to an application of the result of the foregoing ob- 

 servations to the method of viewing the sun advantageously, with 

 telescopes of large apertures and high magnifying powers. It is well 

 known, that even with the naked eye, and much less with telescopes 

 of large apertures, the sun cannot be viewed without a darkening 

 apparatus. Dr. Herschel had repeatedly used red glasses for this 

 purpose ; but always found, that though the lustre was thereby suf- 

 ficiently abated, yet they did not prevent an irritation on the eye, 

 which was manifestly the effect of heat. This induced him to make 

 experiments with glasses of various other colours, the result of which 

 was, that, as might have been inferred from the above investigation, 

 dark-green glasses are the most efficacious for intercepting the red 

 or more calorific rays, and will therefore answer one of the purposes 

 of the darkening apparatus. But as in viewing the sun we have, 

 besides the heat, also its splendour to contend with, further trials 

 were made to obviate this inconvenience : and for this purpose, com- 

 mon smoked glasses were found the most efficacious. Some direc- 

 tions are here given for smoking glasses uniformly, and in the most 

 convenient manner. 



From a series of telescopic experiments respecting this darkening 

 apparatus, which concludes the paper, we learn, that as the heat will 

 often crack the glasses when placed at or near the focus of the pencils 

 of rays, a safer and more advantageous way of applying them is to 

 place them before and near the small speculum, or immediately be- 

 hind the second eye-glass. In this last position a single dark-green 

 glass was found of great utility ; but a deep blue glass, with a blueish- 

 green smoked one upon it, was still preferable : the sun appeared of 

 a whiter colour than with any other composition, and the sensation 

 of heat was by no means inconvenient. 



