SCIENCE. 



591 



the colours in the first prismatic image is almost the 

 same as that produced by a prism of flint glass with a 

 refracting angle of W) . Hence it is obvious, that 

 \\hen nnv el these grooved surfaces is seen where 

 there are a mnnhcr ol lights, the eye must be con- 

 stantly .-tn.v k with one or more of these prismatic 

 images shining with all the hues of the rainbow. In 

 day light, however, the colours are very faint, unless 

 the eye catches the reflexion of some boundary of 

 light and shadow, just as the prism exhibits no colours 

 unless in similar circumstances. 



Air. Barton has already applied this discovery to 

 the manufacture of buttons with great success. The 

 lines or grooves are not drawn by the engine upon 

 each button that is mcde, but a steel die is ibrint d by 

 the engine ; and when the die is properly hardened it 

 is used to impress the grooves upon buttons of brass 

 or any other metal. 



We have before us various specimens of these iris 

 ornaments, containing grooves from 250 up to 10,000 

 in an inch ; and we do not scruple to say that we 

 have never seen any production of the mechanical 

 arts at all to be compared with them in point of 

 beauty and fine workmanship. 



Mr. Barton has mentioned to us a curious experi- 

 ment which he made in drawing these grooves on 

 rock crystal. When he had taken the rock crystal from 

 the engine, he could perceive no traces of his work, 

 which is only seen on metallic surfaces. Even by the 

 aid of a microscope we found it impossible to discover 

 any roughness or diminution of polish, although the 

 whole surface was. covered with grooves in two di- 

 rections transverse to each other, and at the distance 

 of the 2000clth part of an inch. As soon, however, as 

 w* expose it to the sun, or to the light of a candle, the 

 prismatic images on each side of the ordinary image 

 immediately indicate the existence of the invisible 

 grooves. 



The colours produced by these ornaments may all 

 be impressed upon wax, in the same manner as those 

 of mother-of-pearl; (See OPTICS, vol. xv. p. 56p.) and 

 we have even succeeded in communicating the co- 

 lours of striated surfaces from one piece of wax to ano- 

 ther piece of wax, and from the second piece to a 

 third piece. 



8. On a remarkable Change of Colour in Grinding Mer- 

 curial Salts. 



In our article OPTICS, Vol. XV. p. 622, 62.3. we 

 have given an account of several very curious experi- 

 ments, in which certain bodies undergo remarkable 

 changes of colour, by the influence of time, of heat, 

 of moisture, or of rapid cooling. To these we are now 

 enabled to add another fact not less interesting. 



When Mr. Herschel was preparing mercurial salts 

 for his enquiries into the habitudes of the hyposul- 

 phurous acid, he took a quantity of the crystallized pro- 

 to-nitrate of mercury, which is formed when dilute 

 nitric acid is allowed to remain in a moderately warm 

 temperature on excess of metallic mercury. The ac- 

 tion of water resolves this on a super and subsalt. 

 When the crystals were ground in a g!ass mortar, nit/i 

 repeated fffntiotis a/' distilled water, the pctvder preset v- 

 ea its brilliant nhttenrss till the third or fourth ajj'usion, 

 when suuiitnly while grinding, and mixwg it nitlh fresh 

 naler, it pasted aimost in an instant to a sombre greenish 

 yelif.w hue. In continuing the grinding, the colour 

 brightened,' but having desisted for a few moments, 



Mr. Herschel was surprised to find, on 



grinding, that ike yellow green colour had again duap- CmiaaMss 

 jieared, the powder having passed to a light ash grey, ** 

 almost white, and having apparently become more * 

 bulky and crystalline. Mr. Ilercchel found that the 

 powder bad undergone no change in its. chemical pro. 

 ies, dissolving readily in dilute nitric acid, and 

 affording a solution similar in all respects to that ob- 

 tained by water in the course of the working and 

 grinding. 



" As the quantity operated on," says Mr. Herschel, 

 " was rather considerable ( perhaps 2 oz ) and the change 

 of colour simultaneous over the whole mass, there is 

 little doubt of a sudden subversion of equilibrium, and 

 a new arrangement of the molecule* accompanying the 

 phenomenon, though why it should take place at this 

 precise epoch, seems difficult to explain, the abstrac- 

 tion of the acid having been going on gradually from 

 the beginning." See Edinburgh PhiL Journal, vol. 

 ii. p. 155, 



9- Description of a Nciv Compound Prism for Optical 

 Experiment*. 



The difficulty of procuring glass free of veins and 

 striae for the formation of glass prisms, and large thick 

 lenses for burning glasses, has been long known and " 

 severely felt. We have already shown in our article 

 BUHNING Instruments, Vol. V. how this evil 



optical n- 

 may perimemt. 



be remedied in a burning glass, by building it up 

 as it were of different zones or rings, and making each 

 zone out of separate segments. The very same prin- 

 ciple of construction may be applied to the format-on 

 of a prism. 



Let ABC, Plate CCCCLXXXVI. Fig. 18. be the ,, tAT 

 section of a common prism, then it may be easily CCCCIXXXTI. 

 shown that the very same effect may be produced by Fig. IB. 

 six small prisms arranged as shown at AD. It would 

 not be easy to grind these prisms out of a solid piece 

 of glass AD, but they may be all ground in one pris- 

 matic rod, which may be cut into any number of small 

 prisms when polished. If the upper and under sur- 

 faces of the rod are parallel, they may be easily ad- 

 justed so as to have their refracting surfaces parallel. 



If AD is one inch and the depth of the prism one 

 inch, then the quantity of glass in the common prism 

 ABC, will be .5000 of a cubic inch of glass, whereas 

 the compound prism will contain only .01388 of a 

 cubic inch, or ^.th part of the glass of the common 

 prism. In using such a prism, therefore, it is obvious 

 that a much more perfect spectrum must be obtained 

 than can be formed by the prism of tl:e common shape. 



10. Notice of tno kinds of Paradoxical Lenses. 



The following construction has been given by Dr. Node* 01 

 Brewster for two kinds of paradoxical lenses : 



1 . A lens which is at the same lime a plain one, a con- . 

 vex one, and a concave one. If we combine together 

 two lenses of exactly the same curvature, the one con- 

 vex and the other concave, formed cut of two media, 

 which refract the yellow rays equally, while the one 

 refracts the blue rays more than the other, and the red 

 rays less, then such a lens will be a plain one for the} el- 

 low rays, having the same effect upon them as a piece 

 t>f plane glass, while it will be a convex one for the 

 blue rays, it the convex imdium act- in >t power- 

 fully on the blue rays, and a concave one lor the mi 

 rays. 



2. A lens rchich at the same time renders 



