Page:Edinburgh Review Volume 59.djvu/172

160 is no doubt the consequence of the exclusion of diagrams; for it is impossible, by any description, however vivid or circuitous, to convey to the mind a just idea of the exquisite symmetry which characterises the movements of vibrating solids, and which depends on the division of the body by nodal and neutral lines marking the points and lines of repose, and separating the segments which are in a state of depression, from those which are in a state of elevation. There is no branch of physics which addresses itself so agreeably to the eye, or appeals with such force to our wonder, as that of acoustic figures; and, connected as it is with the theory and practice of music, we must implore Mrs Somerville to give it, in another edition, a more favourable consideration. The delineations of Chladni and Savart are highly interesting; and the beautiful analysis of the whole class of phenomena which Mr Wheatstone has given in the last Part of the 'Philosophical Transactions,' and illustrated with about 200 or 300 figures, gives a fresh importance to this branch of physics.

Mrs Somerville next proceeds to the science of Physical Optics, in which she treats, in the compass of eighty pages, of the subjects of inequal refraction, the composition and decomposition of light, accidental colours, the interference of rays, the colours of thin plates, the undulatory theory, and the polarisation and double refraction of light; all of which she discusses with her usual ability, and with the same correct knowledge of the subject which she displays in her own more peculiar department of Astronomy.

Among the more recent enquiries on the subject of light, Mrs Somerville gives a brief account of those of M. Plateau of Brussels, the ingenious improver of that beautiful instrument called the Phenakistiscope. The subject to which we refer is that of accidental colours, respecting which the Flemish philosopher is supposed to have made some important discoveries.

'Recent experiments by Plateau of Brussels prove that direct and accidental colours differ essentially. From these it appears that two complementary colours from direct impression, which would produce white when combined, produce black, or extinguish one another by their union, when accidental; and also that the combination of all the tints of the solar spectrum produces white light if they be from a direct impression on the eye, whereas blackness results from a union of the same tints if they be accidental. M. Plateau attributes the phenomena of accidental colours to a reaction of the retina after being excited by direct vision. When the image of an object is impressed on the retina only for a few moments, the picture left is exactly of the same colour with the object, but in an extremely short time the picture is succeeded by the accidental image. If the prevailing impression be a very strong white light; its accidental image is not black, but a variety of colours in