Page:Philosophical Transactions - Volume 095.djvu/46

This page has been proofread, but needs to be validated.
Dr. Herschel's Experiments on the Means

the art of making them perfectly round and extremely small. To prevent my seeing them at a distance in a different aspect from that in which they were measured under the microscope, I fixed the needles with sealing-wax on small slips of cards before the measures were taken.

Eight of these globules of the following dimensions ,0466 ,0325 ,0290 ,02194 ,0210 ,0169 ,0144 ,00763 were placed upon the post in my garden, and I viewed them in the telescope.

With a power of 231,8 I saw all the first seven numbers well defined, and round, and could see their gradual decrease very precisely from No. 1 to No. 7.

With 303,5 I saw them better, and had a glimpse of No. 8, but could not be sure that I saw it distinctly round; though the magnified angle was 3' 18",2.

With 432,0 they are all very palpable objects, and, as a solid body. No. 8 may be seen without difficulty; at the centre of the mirror it subtends an angle of 0",653. With attention we may also be sure of its roundness; but here the magnified angle is not less than 4' 42",1.

With 522,3 I s^^ them all in great perfection as spherical bodies, and the magnitude of No. 7 may be estimated in quartern of its diameters. The angle is 1",253, and one quarter of it is 0",313. No. 8 may be divided into two halves with ease; each of which is 0",327.

With 925,4 I saw No. 8 still better; but sealing-wax is not bright enough for so high a power.

By. this experiment it appears, that with a globule so small as ,00763 of a substance not reflecting much light, the magnified angle must be between 4 and 5 minutes before we can see it round. But it also appears that a telescope with a sufficient