678 PHILOSOPHICAL TRANSACTIONS. [ANNO 1800. 



my present purpose ; which only goes to prove, that the heating power of the pris- 

 matic colours, is very far from being equally divided, and that the red rays are 

 chiefly eminent in that respect. 



Experiments on the Illuminating Power of Coloured Rays. — In the following 

 examination of the illuminating power of differently-coloured rays, I had 2 ends 

 in view. The first was, with regard to the illumination itself; and the next, with 

 respect to the aptness of the rays for giving distinct vision ; and, though there did 

 not seem to be any particular reason why these 2 should not go together, I judged 

 it right to attend to both. The microscope offered itself as the most convenient 

 instrument for this investigation ; and I thought it expedient to view only opaque 

 objects, as these would give me an opportunity to use a direct prismatic ray, with- 

 out running the risk of any bias that might be given to it, in its transmission 

 through the colouring particles of transparent objects. 



Exper. 1 . I placed an object that had very minute parts, under a double micro- 

 scope ; and, having set a prism in the window, so as to make the coloured image 

 of the sun stationary on the table where the microscope was placed, I caused the 

 differently-coloured rays to fall successively on the object, by advancing the micro- 

 scope into their light. The magnifying power was 27 times. In changing the 

 illumination, by admitting a different colour, it always becomes necessary to re- 

 adjust the instrument. It is well known, that the different refrangibility of the 

 rays will sensibly affect the focal length of object-glasses ; but, in compound 

 vision, such as in a microscope, where a very small lens is made to cast a length- 

 ened secondary focus, this difference becomes still more considerable. By an 

 attentive and repeated inspection, I found that my object was very well seen in 

 red ; better in orange, and still better in yellow; full as well in green; but to less 

 advantage in blue; indifferently well in indigo, and with more imperfection in 

 violet 



This trial was made on one of the microscopic objects generally prepared for trans- 

 parent vision: but, as I used it in the opaque way, I thought that others might be 

 chosen which would answer the purpose better ; and, in order to give some variety 

 to my experiments, and to see the effect differently coloured substances might have 

 on the rays of light, I provided the following materials to be viewed. Red paper ; 

 green paper ; a piece of brass ; a nail ; a guinea ; black paper. Having also found 

 that a higher power might be used, with sufficient convenience for the rays of 

 light to come from the prism to the object, I made the microscope magnify 

 42 times. 



The appearance of the nail in the microscope, is so beautiful, that it deserves 

 to be noticed; and the more so, as it is accompanied with circumstances that are 

 very favourable for an investigation, such as that which is under our present con- 

 sideration. I had chosen it on account of its solidity and blackness, as being most 

 likely to give an impartial result, of the modifications arising from an illumination 

 by differently-coloured rays; but, on viewing it, I was struck with the sight of a 



