230 Dr. Herschel’s Observations of the Transit of Mercury , 
and caused an indistinctness of vision, which plainly indicated 
that, under such circumstances, the magnifying power of the 
telescope, 225, was more than it ought to be, in order to see 
well. 
3d Experiment. 
I now changed the glass mirror for a metalline one ; and, on 
placing the heater near the back of it, the focus of the speculum, 
in 30 seconds, became ,77 inch shorter. But, continuing the 
observation, instead of shortening still farther in the next 30 
seconds, it became ,3 inch longer, so that, at the end of a minute, 
it was only ,47 shorter than before the approach of the hot iron.. 
4 th Experiment . 
When the small heated globe of the 2d experiment was 
suspended in front of the mirror, the focus lengthened ,27 inch 
in one minute ; nor would the lengthening increase by leaving 
the hot iron longer in its position. The foci in this, as well as 
in the 3d experiment, were so much injured that they could not 
be measured with any precision ; and it was evident, that high 
magnifying powers ought not to be used with a mirror of which 
the temperature is undergoing a continual change. 
I repeated the experiment with the iron nearly red hot ; and 
found the focus lengthened 1,48 inch in 30 seconds. Five 
minutes after the removal of the iron, the regularity of the 
figure of the mirror was pretty well restored. 
With a moderate heat, I had, in 30 seconds, a lengthening of 
the focus, of ,57 inch ; and, in about minute after the removal 
of the heated iron, distinct vision was nearly restored. 
These four experiments show, that a change in the tempera- 
ture of mirrors, occasioned by heat, is attended with an alteration 
