S2 REPORT — 1861. 



tion of the polish, reflect very little light, was set down to some yet undiscorered 

 disturbance. 



The last question noticed was the apparent increase in the size of the sim and 

 moon when near and upon the horizon, which was illustrated by a description of 

 an experiment, which consists in looking at a ball suspended by a fine silken thread, 

 both when the external light does and does not fall upon the side next to the spec- 

 tator, in the latter of which cases the baU appears larger than it would if looked 

 upon in the hand ; from which it was concluded that this supposition arises from 

 its being supposed to be at a greater distance from the spectator than is really the 

 case, and this consequent upon its dullness ; and this explanation was applied to 

 the sun and moon when in the positions mentioned, as being at any rate one cause 

 of the phenomenon which may then be obsei-ved. 



Presentations of Colour produced under novel conditions ; with their assumed 

 relation to the received Theory of Light and Colour. By Thojias Eose. 



The author succeeded some years ago in perfecting a mechanical contrivance for 

 measming off flashes of artiflcial light, in due relation to the velocity of an inde- 

 pendent revolving disc. 



This apparatus was originally designed for no higher pm-pose than showing the 

 ordinary, yet remarkable illusions of persistence of vision to a large company. Acci- 

 dent led to its employment in the illustration of phenomena of greater interest. It 

 was foimd that a disc charged with eight intensely black cu-cular spaces, equi- 

 distantly aiTanged around its circumference, presented some noticeable eflects when 

 subjected to the action of continuous daylight and intei-mittent aitificial light. After 

 repeated and carefid experiment, it was ascertained that if, whilst the disc is in 

 rapid revolution under a weak continuous daylight, flashes of artificial light are 

 thrown upon it in rapid and regidar succession, and at such intervals that the black 

 circidar spaces be held at apparent rest, several varieties of positive colour are seen. 

 The black spaces show an intense blue in the central parts, melting towards the inner 

 circumference into lighter blue, and towards the outer circiunference into gi-een ; and 

 they appear to lie upon a zone presenting intense orange in the centre, and lighter 

 orange and yellow at the inner and outer circimiferences. Other discs, in which the 

 black cii'cidar spaces vary in diameter and occupy lesser portions of the zones con- 

 taining them, were obsei-ved to give modified analogous eflects. There was an 

 evident law in the action. The colours were obviously dependent on the relative 

 amounts of black space and white surface in the zones. As the black spaces were 

 reduced, the colom-s ranged from dark blue to light gi-een, and the sepai-ating inter- 

 vals of whiteness, of greater or lesser width, took all tints, from intense orange to 

 the faintest yellow. 



The author deemed it worthy of especial remark, that aU these presentations of 

 colour were produced at pleasure under uniform conditions of action, and that they 

 were so sti-ongly and unequivocally expressed as to afiect all eyes alike. This he 

 thought made sepai'ation between them and other colour-eflects that are merely 

 physiological phenomena. 



_ He was thus led to assimie that his experiments touched the question — 7s light 

 simple or compound ? and after much thought, it did appear to him that the result- 

 ing phenomena found consistent explanation in the assimied homogeneity of light, 

 but presented difliculties when brought into relation with the received doctrine. 



By a variety of experiments, earned on over a period of more than six years, Mr. 

 Rose had been brought to favour the idea that what we name coloiu', is only the 

 various afiections of the optic nerve by a greater or a lesser quantity of light radiating 

 from a focal point in an maperfect reflector. It is obviously impossible on this oc- 

 casion to ti-ace the steps by which he was led to form this conclusion. All that can 

 be permitted, is to state briefly, and in general, the application of his views to the 

 phenomena under consideration. When the disc is m rapid revolution, the weak 

 continuous daylight keeps it constantly before the eye, but the intermittent light 

 presents the black spaces continually in the same areas. Now the black spaces are 

 assximed to have no pai-t in the phenomena, except as absolute negations of light, 

 and aU the effects are referred to the distribution over an entire zone of the light of 

 those portions of the zone not occupied by the black spaces. The nebulous ring 



