of the prismatic Colours to heat and illuminate Objects. 263 



sensibly affect the focal length of object-glasses ; but, in com- 

 pound vision, such as in a microscope, where a very small lens 

 is made to cast a lengthened 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 imper- 

 fection in violet. 



This trial was made upon one of the microscopic objects 

 which are generally prepared for transparent 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 miscroscope 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 accom- 

 panied with circumstances that are very favourable for an inves- 

 tigation, such as that which is under our present consideration. 

 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 bright constella- 

 tion of thousands of luminous points, scattered over its whole 

 extent, as far as the field of the microscope could take it in. 



mdccc» M m 



