d
70 O B S E R V A T I O N S ON THE
coolly coniidered every circumftance, I began to fufpeft feme optical deception,
though I could not fufficiently explain the caufes of it.
In ftiort, to give the reader an idea of the diilinflnefs of the appearance,
or its feeming reality, and, at the fame time, to give him a convincing
px'oof of the difficulty of finding optical principles by which the caufes of
fo great a deception can be explained, I need only to mention, that, after
I had extended my obferyations to the vegetable and mineral kingdoms,
and had had time to confider every circumftance, and, particularly, had remarked
that the convolutions feemed to change place, by altering the dire
«3;ion of the light, in a greater degree than could be accounted for from
th& mere alteration of the lhades, if the fibres were real and fixed in their
place, and that I had written, on April ill 1779, the grounds of my doubts
to one of the firft fpeculative as well as pradical opticians in Europe, he
anfwered my letter in the following words:
' I have examined the different kinds of fubftances mentioned in your
' letter to be compofed of Terpentine fibres, and fee it clearly in human hair,
' but as to the reft, am in fome doubt; but, perhaps, I have not tried it
' fufficiently, my time being very much taken up in bufinefs. I cannot
' think that the appearance of thefe fibres can be any optical deception,
' but their fhifting may be fo, though it is not the fibres only which feem
' to move, but the whole objed, and I think it may be owing to the dif-
< ferent refrangibility of light; for the more the aperture of the obje£l lens
' be opened, the more this will appear. The fmall circles of aberration
' occafioned by the refrangibility, appear different, as the light is thrown
* in different diredtions, and the eye may be more affedbed by fome colours
' in one cafe, and by others in another.
' I have the honour to be,
' Sir, your obedient fervant.'
When,
N E R V O U S S Y S T E M . 71
When, however, all circumftances are duly weighed, we muft, I apprehend,
be led to conclude, that this very curious form, in which bodies
viewed by the microfcope appear, is produced by an optical deception ; for,
independent of the improbability that the particles of all kinds of matter
ihoot into fimilar and vifible forms, I found, on accurate infpedion,
1. That, in viewing hairs, and other objeds, the ferpentine turns feemed
more numerous, as well as more diftind, when the light did not fall
perpendicularly upon the objed, but in a flanting diredion.
2. la the next place, although I obferved, that, when I placed a fmall
bunch of ferpentine woollen threads before the microfcope, and threw the
light upon them in different diredions, the fibres beyond the focus feemed
to change place, yet the number and diredion of the turns at the focus remained
the fame; whereas, in the appearance of ferpentine fibres I have
been defcribing, the number, fituation, and diredion of turns was greatly
altered by altering the quantity and diredion of the light.
I thought, at firftj after I obferved the illifting of place in the ferpentine
turns, that this might be accounted for from the difficulty, or impoffibility,
efpecially where a microfcope of high magnifying powers is employed,
of bringing all the fibres to the exaft focus, and that this apparent
illifting might be fimilar to the fhifting I had obferved in the bunch of
woollen threads. But I afterwards found that the apparent Qiifting is
greater than could be accounted for in that way; and my very ingenious
colleague Mr John Robifon, Profeflbr of Natural Philofophy, who was fo
obliging as, at my defire, to examine this fubjed, is of opinion, that the
appearance of thefe convoluted fibres is an optical deception, arifmg partly
from the unavoidable aberration of light in the microfcope, and partly
from the irregular difperfion of it from bodies, whofe inequalities are neither
incomparably greater nor incomparably fmaller than the diftance at
which light is adled on by bodies.
I