Page:Philosophical Transactions - Volume 104.djvu/258

Rh made by cutting circular holes in two slips of black card paper, and covering them with fine oiled paper. Small shades of black card paper were contrived to shield the disks from the direct rays of the candle. The disk within the focus being made the brightest, was moved at intervals towards the mirror till the observer pronounced both disks to be equally illuminated, when their distance was registered as the "first reading." The disk within the focus was then advanced so near the mirror, as to appear much the faintest, and afterwards brought back gradually till the illumination of the disks was a second time pronounced to be equal, when their distance was again measured and entered under the head of "second reading," and the same method was followed in all the subsequent experiments.

With large transparent disks.

Distance of the candle from the mirror 186 inches.

Observers.

Distance of the disks when equally illuminated.

Distance of the disk from the focus without.

Distance of the disk from the focus within.

Ratio of the intensity of the light at equal distances from the focus.

1st reading.

2d reading.

Within.

Without.

Myself 26,45 25,70 10, 3 15,77 1000 427

K 24, 5 25, 6 10,22 14,83 1000 475

Mean 1000 451

In making the third set of experiments I remarked that a part of the light from the mirror was received on that portion of the card which was blackened, and fearing that this circum-