David Brewster

XXIV.—On the Law of Visible Position in Single and Binocular Vision, and on the representation of Solid Figures by the union of dissimilar Plane Pictures on the Retina.

In the course of an examination of Bishop Berkeleys “New Theory of Vision,” the foundation of the Ideal Philosophy, I have found it necessary to repeat many old experiments, and to make many new ones, in reference to the functions of the eye as an optical instrument. I had imagined that many points in the physiology of vision were irrevocably fixed, and placed beyond the reach of controversy; but though this supposition may still be true in the estimation of that very limited class of philosophers who have really studied the subject, yet it is mortifying to find that the laws of vision, as established by experiment and observation, are as little understood as they were in the days of Locke and Berkeley. Metaphysicians and physiologists have combined their efforts in substituting unfounded speculation for physical truth; and even substantial discoveries have been prematurely placed in opposition to opinions of which they are the necessary result.

XLIII.—On the Knowledge of Distance given by Binocular Vision

In analysing Mr Wheatstones beautiful discovery, that in binocular vision we see all objects of three dimensions by means of two dissimilar pictures on the retina, I trust I have satisfied the Society that the dissimilarity of these two pictures is in no respect the cause of our vivid perception of such objects, but, on the contrary, an unavoidable accompaniment of binocular vision, which renders it less perfect than vision with one eye. On the other hand, it is quite true that, in Mr Wheatstones experiment of producing the perception of objects of three dimensions by the apparent coalescence of two dissimilar representations of such objects in plano , the dissimilarity of the pictures is necessary in the exhibition of that beautiful phenomenon.

XIX. On the Colours of Natural Bodies

There are few of the applications of optical science so universally interesting as that which has for its object the explanation of the colours of natural bodies. Sir Isaac Newton was the first person who ventured to refer to one general principle all the variety of colours which are found in nature; and he has maintained his opinions on this subject with a confidence in their accuracy which seems to have confounded his adversaries: For while his analysis of light, the most perfect of all his labours, exposed him to the most harassing controversies, his theory of natural colours, the least perfect of his speculations, was allowed to pass without examination or censure.

XIII.—On the Decomposition and Dispersion of Light within Solid and Fluid Bodies

Hauy, and other mineralogists, observed the two colours which are visible in several varieties of fluor-spar. He regarded the two tints as complementary, and explained them, as he did every other analogous phenomenon, by a reference to the colours of thin plates. In describing a species of dichroism noticed by Dr Prout in the purpurates of ammonia and potash, Sir John Herschel ascribes the green reflected lights “to some peculiar conformation of the green surfaces producing what may be best termed a superficial colour , or one analogous to the colour of thin plates, and striated or dotted surfaces.” And he adds—“A remarkable example of such superficial colour, differing from the transmitted tints, is met with in the green fluor of Alston Moor, which, on its surfaces, whether natural or artificial, exhibits, in certain lights, a deep blue tint, not to be removed by any polishing.”

III.— On Hemiopsy, or Half-Vision

The affection of Half-vision, or Half-blindness as it has been called, was first distinctly described by Dr Wollaston, in a paper “On Semidecussation of the Optic Nerves,” published in the Philosophical Transactions for 1824. “It is now more than twenty years,” he says, “since I was first affected with this peculiar state of vision, in consequence of violent exercise I had taken for two or three hours before. I suddenly found that I could see but half the face of a man whom I met, and it was the same with every object I looked at. In attempting to read the name Johnson over a door, I saw only son, the commencement of the name being wholly obliterated from my view. In this instance, the loss of sight was towards my left, and was the same, whether I looked with my right eye or my left. This blindness was not so complete as to amount to absolute blackness, but was a shaded darkness, without definite outline. The complaint lasted only about a quarter of an hour.” In 1822, Dr Wollaston had another attack of hemiopsy, with this difference, that the blindness was to the right of the centre of vision, and he has referred to three other cases among his friends; but in these, the affection was accompanied with headache and indigestion.

VI. Description of Hopeite, a New Mineral, from Altenberg near Aix-la-Chapelle

About the end of the year 1821, when I was engaged in the examination of the Family of the Zeolites, Mr Heuland was so obliging as to send me a variety of Stilbite from a Calamine mine at Altenberg near Aix-la-Chapelle. Upon comparing its optical structure with that of the Stilbites, it was manifest that it had no connection with this class of crystals, and that it constituted a new mineral species. Upon mentioning this result to Mr Brooke, this acute mineralogist was of opinion that it was the Silicate of Zinc. Mr Heuland had been led to regard this substance as a Stilbite, in consequence of having received it as such from Major Petersen; but, particularly, from finding in the collection of Mr C. H. Turner, a single crystal of the same substance attached to Carbonate of Zinc, and bearing the annexed figure, with an inscription in the handwriting of the Abbe Hauy, stating it to be a new variety of Stilbite, to which he gave the name of Stilbite Duovigesimale .

XLIII.—On Circular Crystals

In 1836, Mr Fox Talbot communicated to the Royal Society a paper “On the Optical Phenomena of certain Crystals” which he obtained by dissolving a crystal of Borax in a drop of somewhat diluted Phosphoric acid. When the acid and the salt are in proper proportions, “the field of view of the microscope is seen covered with minute circular spots, each of which is like a tuft of silk radiating from a centre, and is composed of a close assemblage of delicate acicular crystals forming a star.” Among these crystals are seen interspersed “a number of circular transparent bodies, which are tufts or stars of acicular crystals, in such close assemblage as to be in optical contact with each other, and to produce the appearance of a single individual.”

XIX. On certain new Phenomena of Colour in Labrador Felspar, with Observations on the nature and cause of its Changeable Tints

Sir Isaac Newtons theory of the colours of natural bodies, is perhaps the most ingenious and lofty of all his speculations. It was devised, however, at a time when the doctrine of light had made comparatively but little progress, and before the discovery of various principles on which the colours of bodies must depend, or by which, at least, they must be extensively modified. The different dispersive powers of transparent substances;—the irrationality of the spectrum;—the action of striated surfaces;—the decomposition of polarised light;—the reflection of coloured light at the confines of equally refracting media;—and the absorption of common and of polarised rays,—are principles which embrace within their individual range a great variety of facts to which the Newtonian theory of colours bears no relation.