e6a Dr. Herschel's Investigation of the Powers 



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 two 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 two should not go together > I judged it right to attend to 

 both. 



The microscope offered itself as the most convenient instru- 

 ment 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, without running the risk of any bias 

 that might be given to it, in its transmission through the colour- 

 ing particles of transparent objects. 



ist Experiment. I placed an object that had very minute 

 parts, under a double microscope ; and, having set a prism in the 

 window, so as to make the coloured image of the sun stationary 

 upon the table where the microscope was placed, I caused the 

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

 advancing the microscope 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 



