determine the first- mentioned question, made a series of experiments.
of which the following is a summary account.
Dr. Herschel’s first experiment was made upon the heads of pins, the size of the largest of which was '1375, and that of the smallest '0425 of an inch. These pins were placed at the distance of 2407-85 inches from the centre of the object-mirror of the author’s ten-feet telescope; the focal length of the mirror of which, on Arcturus, is 119'64 inches, but on the objects above mentioned 125'9.
Dr. Herschel soon found that none but the smallest of those objects could, at the distance at which they were placed, be of any use ; and that an object of that size, namely, '0425 of an inch, may be easily seen, in his telescope, to be a round body when the magnified angle under which it appears is 2' 18"'9, and that with a high power a part of it, subtending an angle of 0"'364, may be conveniently per- ceived.
In the second experiment, Dr. Herschel made use of globules of sealing—wax. These globules were of different sizes, from '0466 to '00763 : and the result of the experiment was, that, with a globule so small as '00763 of an inch of a substance not reflecting much light, the magnified angle must be between 4 and 5 minutes before we can perceive it to be round. But it also appears that a telescope, with a sufiicient power, will show the disc of a faint object when the angle it subtends, at the naked eye, is no more than 0"‘653.
The third experiment was made with globules of silver, formed by running the ends of very fine silver wires into the flame of a candle. The size of these globules was from '03956 to '00556, and the di. stance of these objects from the mirror of the telescope was increased to 2370'5 inches. By this experiment it was found that the telescope acted very well with a high power, and would show an object, sub- tending only 0"‘484, so large, that it might be divided into quarters of its diameter.
The fourth experiment was made with globules of pitch, bees’ wax, and brimstone. From the two first-mentioned substances, no satis- factory inference could bc drawn; but when four globules of brim- stone, the sizes of which Were '00962, ‘009125, '00475, '002375, were viewed with a power of 5227, the three first appeared round; the fourth was invisible till a dark blue paper was placed a few inches behind it. The angle it subtended was 0"‘207.
The fifth experiment consisted in observing the globules of sealing wax and those of silver at a distance still greater, namely, 9620'4 inches, with a power of 502 : the smallest globules of the former substance were invisible at that distance ; but all the silver globules except the smallest (which, having met with an accident, could not be examined,) appeared round.
The sixth experiment was made by fixing some of the silver globules on a post, and illuminating them, by holding a lantern against them : with a power of 522-7 they were all seen perfectly well, but the light thrown on them was not sufficient to allow of making angular experiments upon them.
