Dr. Herschel then, in order to investigate the causes of the spul
rious diameters of celestial objects, relates some observations made
upon several of the fixed stars, from which he deduces the following
inferences :—
1. That the spurious diameters of the stars are larger than the real ones, which are too small to be seen.
2. That the spurious diameters are of diferent sizes; but that, under the same circumstances, their dimensions are of a pennanth nature.
3. That the spurious diameters are differently coloured, and that these colours are permanent when circumstances are the same.
4. That these diameters are proportionally lessened by increasing the magnifying power, and increase when the power is lowered.
5. That the above-mentioned increase and decrease is not inversely as the increase and decrease of the magnifying power, but in a much less ratio.
6. That the magnifying power acts unequally on spurious diame- ters of different magnitudes; less on the large diameters, and more omhe small ones.
7. That when the aperture of the telescope is lessened, it occa- sions an increase of the spurious diameters; and when increased, reduces them.
8. That this increase and decrease is not proportional to the dia- meters of the stars, but that an alteration in the aperture of the te- lescope acts more upon small spurious diameters and less upon large ones. -
9. That stars which are extremely small lose their spurious dia- mcters, and become nebulous.
10. That many causes will influence the apparent diameter of the spurious discs of the stars ; but that, with a proper regard to those causes, the conclusion already drawn, that under the same circumv stances their dimensions are permanent, still remains good.
A number of experiments then succeed on the spurious diameters of terrestrial objects. The first series of these were made by means of the silver globules already mentioned. The inferences drawn from them are similar to those drawn from the observations of celestial objects, except that the spurious discs of terrestrial objects, contrary to what happens with celestial ones, are smaller than the real discs, and that they are apt to be lost for want of proper illumination, but do not on that account change their magnitude.
Similar experiments were made with drops of quicksilver, the re- sults of which differed so little from those of the experiments with globules of silver, as not to require any further description of them.
After two preparatory experiments, one with black and white circles, which showed that no material deception can take place in estimating by such circles, on account of their colour, and another,- which showed that no difiemnce in the apparent size of the globules was produced by a different mode of illuminating them in the micro- scope, Dr. Herschel proceeded to measure the spurious disc of one
