144 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



still smaller image, scarcely visible, in fact, with the one twen- 

 ty-fifth screen in which the proportion of light and darkness 

 is well marked, being better marked with the finer screens. 

 In the intermediate distances we have upon our screen no 

 image, but simply a luminous spot. If we repeat this exper- 

 iment with white light, we find the circular net acting like 

 a non-achromatic and very dispersive lens. At the distance 

 proper for the red rays, the image is red, surrounded by a 

 blue aureole. On removing: the screen further, the image 

 changes through the yellow and the green, and finally be- 

 comes blue ; having in the latter case a red aureole. From 

 this latter experiment Soret shows that w r e can consider the 

 little circular screens as concave lenses, so that the ordinary 

 Galilean telescope may be constructed by employing such a 

 screen instead of the eye-glass. Bull. Hebdom., XVI., 11. 



THE COLOR OF DIAMONDS. 



Flight and Maskelyne have lately made some curious ob- 

 servations upon colored diamonds. It has for some time 

 been known that the tints of these stones are either destroy- 

 ed or modified by heating, the change being sometimes tem- 

 porary, sometimes permanent. In the present case two yel- 

 lowish diamonds from the Cape of Good Hope were strongly 

 heated in an atmosphere of hydrogen in a porcelain tube, for 

 about two hours. Upon cooling, the color of the stones was 

 found to have vanished, but it returned after exposure, for 

 only a few minutes, to diffused light. In one instance a dia- 

 mond which had been decolorized by heat was kept in the 

 dark for three days, and remained colorless; but an exposure 

 of six or seven minutes to the light again brought back its 

 yellowish hue. These facts appear to stand in some relation 

 to phenomena of phosphorescence. 15 C, XXIX., 33. 



GILT GLASS PRISM IX THE CONSTRUCTION OF THE CAMERA- 

 LUCID A. 



By taking advantage of the property possessed by thin 

 metallic films of allowing the passage of direct rays through 

 them, while they reflect oblique rays from some other source, 

 Professor Govi, of Rome, has devised a perfect method for 

 superposing a direct and reflected image, as is necessary in 

 the camera-lucida, without the usual fatigue to the eye. He 



