12 REPORT — 1860. 



pure glass, — deriving their colour from the individual films of which they are com- 

 posed. This is obvious from the fact of their becoming colourless by a sufficient 

 inclination of the plates, and also by the introduction of a drop of water or alcohol. 

 When the fluid has evaporated, the films recover their original colour; and though 

 a film of fluid has separated each of the almost infinitesimal layers of the glass, yet 

 they adhere as firmly as ever after the fluid has evaporated. If an oil or balsam is 

 introduced, it passes' slowly and unequally between the layers, so that the retreating 

 colour is bounded by a spectrum of the various tints which the film combines. 



But though the films themselves are glass, yet I have often found between them 

 beautifid circular crystals of silex, which are finely seen in polarized light, and exhi- 

 bit many of the regular and irregular forms which I have represented in a paper on 

 Circular Crystals lately published in the ' Transactions of the lloval Society of Edin- 

 burgh.' They are sometimes dendritic, and assume, round the black cross, foliated 

 shapes like the leaves of plants. At other times, but very rarely, they occur in 

 circular groups, — related to a crystal of silex in their centre. One of these groups 

 is so remarkable as to merit particular notice. Around a minute speck of silex 

 there is formed, at a considerable distance from it, a circular band of equally minute 

 crystalline specks, and at a greater distance a second circular band concentric with 

 the first, and consisting of still smaller siliceous particles, hardly visible in the mi- 

 croscope. By what atomic force, or by what other cause, the central crystal has 

 placed its attendant crystals in regular circles around it, remains to be discovered. I 

 nave already described a similar phenomenon, as produced during the formation of 

 circular crystals under constraint, and when crystallizing freely ; but I am not aware 

 that any other person has either seen the phenomenon or attempted to explain it. 



The films of decomposed glass, as I have long ago shown, absorb definite rays of 

 the spectrum like coloured media. They change, in the most distinct manner, the 

 colours of different parts of the spectrum, and frequently insulate bands of purely 

 white light, in or near its most luminous division. 



[The drawings referred to in this communication were laid before the Section, and 

 some of the specimens of decomposed glass were exhibited in the Museum in the cotirse 

 of the evening. 1 



On his own Perception of Colours. By J. H. Gladstone, Ph.D., F.R.S. 



The author described himself as in an intermediate position between those who 

 have a normal vision of colours, and those who are termed " colour-blind." These 

 latter are usually unacquainted with the sensations of either red or green, and it 

 becomes a desideratum to have good observations on those who are capable of acting 

 somewhat as interpreters between them, and those who perceive every colour. By 

 means of Chevreul's chromatic circles and scales, Maxwell's colour-top, coloured 

 beads, &c, the author was able to determine the following points in respect to his 

 own vision. He sees red, in all probability, like other people, but it requires a larger 

 quantity of the colour to give the sensation than is usually the case ; hence a purple 

 appears to him more blue, and an orange more yellow, than to the generality of 

 observers. He is perfectly sensible of green, or rather of two distinct greens, the 

 one yellowish, the other bluish ; but between them there lies a particular shade of 

 green, to which his eyes are insensible as a colour. This modifies his perception of 

 many greens that approximate to what is to him invisible. The shade occurs in 

 nature on the back of the leaf of the variegated holly, and it may be produced in Max- 

 well's top by certain combinations of the coloured disc; the simplest being 



94"5 Brunswick Green (Blue Shade) +5 - 5 Ultramarine = 94 Black + 6 White. 



He finds that this shade, though invisible to him as green, is yet capable of neu- 

 tralizing red when viewed simultaneously, but it does not neutralize so much red 

 with him as with observers of oidinary vision. 



While able perfectly to distinguish between red and green, the contrast does not 

 readily catch his eye, especially at a distance ; in fact, he is somewhat short-sighted 

 in respect to these colours. lie has reason to believe that, in his case, there has 

 been a gradual improvement in his actual perception of colours, independently of his 

 greater knowledge of them, though this is in opposition to the general experience of 



