PHYSICS. Ill 



LIGHT. 

 1, Reflection. 



Govi has proposed the use of thin layers of gold-leaf for 

 obtaining good transmitting and reflecting surfaces in op- 

 tical experiments. If upon the oblique face of a right-angled 

 isosceles prism a very thin layer of gold be deposited by 

 means of an alkaline solution of gold chloride and aldehyde, 

 and then the prism be cemented by Canada balsam to a sec- 

 ond and similar prism, a cube is obtained containing in its 

 interior a surface of gold inclined at 45 to two opposite sur- 

 faces. By means of such a cube two images are seen one 

 by light transmitted through the film, which is of a pale 

 green color; the other by light reflected from the film, and 

 which is yellow. The cube thus becomes of excellent service 

 as a camera lucida, etc. Govi proposes to place such a cube 

 on the front of the telescope of a cathetometer, and then to 

 compare directly the object to be measured with the equally 

 distant scale by means of the direct image of the one and 

 the reflected image of the other. 



Wright has continued his researches on the volatilization 

 of metals by the electric spark in vacuo, and has successful- 

 ly applied the method to the production of mirrors. The glass 

 to be metalized is placed in an exhausted globe, and a show- 

 er of sparks rained upon it from the negative electrode made 

 of the metal to be used until the deposit was sufficiently 

 thick. Platinum appears to be the best metal for specula, 

 a perfect coat being deposited on a plate two centimeters 

 in diameter in twenty to thirty minutes, the vacuum for the 

 purpose being from 1.5 to 1.75 millimeters, and made on hy- 

 drogen. This layer was found to be 0.000174 millimeter 

 thick, or one fourth of the length of a wave of red light. The 

 author thinks this process may be brought into general use 

 in the arts, the polish of the metallic surface being exquisite, 

 far surpassing that obtained artificially. 



Miller has described a new form of Wollaston's reflecting 

 goniometer, which appears to him to have numerous advan- 

 tages. There is nothing new in the leading principle in- 

 volved in the instrument, the changes being mainly in the 

 mechanical construction. 



