232 Dr. Herschel’s Observations of the Transit of Mercury, &c. 
is exposed to the sun, every part of the surface will receive an 
equal portion of heat. 
It may also be said, that I have pointed out a defect in tele- 
scopes used for solar observations, without assigning a cure for 
it. It will however be allowed, that tracing an evil to its cause 
must be the first step towards a remedy. Had the imperfection 
of the figure brought on by the heat of the solar rays been of a 
regular nature, an elliptical speculum might have been used to 
counteract the assumed hyperbolical form ; or vice versa. 
And now, as, properly speaking, the derangement of the 
figure of a mirror used in observing the sun, is not so much 
caused by the heat of its rays as by their partial application to 
the reflecting surface only, which produces a greater dilatation 
in front than at the back, there may be a possibility of counter- 
acting this effect, by a contrary application of heat against the 
back, or by an interception of it on the front. But this we leave 
to future experiments. 
