with Dove's Theory of Lustre, 43 



ened that no light is sent from it to the eye, this lustre vanishes,- 

 and the white paper alone is perceived. This disagreement is 

 not a cause of astonishment when we reflect that De Haldat's 

 original experiment waited nearly half a century for confirma- 

 tion. 



To Brewster's own theory, the simple objection, which has 

 already been made by others, that we daily perceive lustre 

 plainly with one eye, would seem sufficient. 



Production of Lustre in Monocular Vision. 



I proceed now to describe some experiments where by the 

 action upon a single eye of two masses of light of unequal in- 

 tensity, the idea of lustre is produced. 



1. If a disk of coloured card-board, out of which a number of 

 sectors has been removed, be made to rotate rapidly, and an 

 object be viewed through it by a single eye, two masses of light 

 will reach the eye, which apparently proceed from the object; 

 one is reflected from the surface of the disk, the other emanates 

 from the object behind the disk, and passes through the first- 

 mass of light. Dark objects viewed in this way assume to me, 

 to a small extent, an appearance like that of blackened glass. The 

 effect is not at all striking, and would be overlooked by many 

 persons ; I therefore prepared paper in a peculiar way, so as to 

 imitate distantly the appearance of foliated graphite or crumpled 

 mica. 



White smooth drawing-paper was rubbed over irregularly 

 with a brush slightly moistened with a weak wash of India ink 

 or lampblack ; when dry, another wash of a deeper hue applied 

 as before, care being taken to leave many small spots untouched. 

 The final wash was laid on with pure black. If the brush be 

 kept nearly dry and passed only lightly over the paper, it is 

 easy to obtain a surface bearing some very distant resemblance 

 to the minerals above mentioned ; it is of course without lustre. 

 Similar papers were prepared with red and blue water colours. 



When these papers were held behind disks of ultramarine or 

 orange-tinted paper, from which equal alternate sectors had been 

 removed, and which were revolving at such rates that their 

 surfaces seemed uniform, or at louver rates, they often appeared, 

 to a single eye, highly lustrous. This was true of the prepared 

 paper in a state of rest; when moved slightly by the hand it 

 glittered strongly. Dark photographs of tinfoil held behind a 

 revolving disk of ultramarine paper and viewed by a single eye, 

 assume often to a striking degree the lustre and appearance of 

 foliated graphite. 



2. If a piece of this peculiarly blackened paper % an inch 

 square be placed in a blue field (rather light ultramarine paper) 



