16 Dr. Young’s Experiments and Calculations , &c. 
rings of violet light that appeared upon the interposition of violet 
glass. I thought the dark rings were a little smaller than the 
violet rings, but the difference was not sufficiently great to be 
accurately ascertained ; it might be as much as —■ or ~ of the 
diameters, but not greater. It is the less surprising that the 
difference should be so small, as the dimensions of the coloured 
rings do not by any means vary at the violet end of the spec- 
trum, so rapidly as at the red end. For performing this experi- 
ment with very great accuracy, a heliostate would be necessary, 
since the motion of the sun causes a slight change in the place 
of the image; and leather, impregnated with the muriate of 
silver, would indicate the effect with greater delicacy. The ex- 
periment, however, in its present state, is sufficient to complete 
the analogy of the invisible with the visible rays, and to show 
that they are equally liable to the general law which is the 
principal subject of this Paper. If we had thermometers suffi- 
ciently delicate, it is probable that we might acquire, by similar 
means, information still more interesting, with respect to the 
rays of invisible heat discovered by Dr. Herschel ; but at pre- 
sent there is great reason to doubt of the practicability of such 
an experiment. 
