INDUSTRIAL PROGRESS DURING THE YEAR 1S7G. Hx 



OPTICS. 



C.W.Siemens has exhibited at the Roj-al Institution some remaiiv- 

 able experiments on the action of light on the conductivity of crys- 

 tallized selenium, mainly the results which had been obtained by 

 Dr. Werner Siemens, of Berlin. He shows that cast selenium be- 

 comes more homogeneous and extraordinarily sensitive to light if it 

 be kept for some time at or above 210 Centigrade. In this condi- 

 tion it may be used as a photometer, and is capable of giving accu- 

 rate and constant results. Diffuse daylight doubles the conductivity, 

 and direct sunlight increases it more than tenfold. A selenium 

 "eye" was shown which had a selenium disk for the retina, and 

 two slides for the eyelids. The eye was shown to be sensitive to 

 white light, and also in different degrees to light of different colors. 

 By means of an easily arranged electro- magnetic arrangement the 

 current might be made to open and close the eyelids automatically, 

 the selenium eye blinking by a flash of light exactly as does the 

 natural one. 



Lommel has given some ingenious and satisfactory demonstrations 

 of certain optical problems not easily made evident without the 

 higher mathematics, such, for example, as the angle of minimum de- 

 viation in prisms, achromatism, theory of the rainbow, etc. 



Allard has investigated several questions of practical interest in 

 light-house illumination, such as the transparency of flames and of 

 the atmosphere, and the visibility of scintillating lights. 



Rutherfurd has published a short account of his new glass circle 

 for angle measurements. The circle is ten inches in diameter, and 

 divided to ten minutes of arc. It is read by two micrometer micro- 

 scopes magnifying seventy-five times. The probable error of a sin- 

 gle reading is shown to be considerably less than half a second, 

 while that of the mean of a series of such measurements is very much 

 less. The author expresses his conviction that upon a fifteen-inch 

 glass circle, provided with powerful microscopes, greater precision 

 could be attained in the reading of angles than with the largest 

 metallic circles now in use. 



Kriiss has studied the question of the depth of the images in 

 optical instruments, and has given the results of the application ol" 

 his principles to the human eye. 



Abneyhas investigated the cojiditions of photographic irradiation 

 which causes the photographic image of a luminous body in front 

 of a dark background to appear larger than it is and concludes 

 that the current theory that it is due to reflection from the back 

 of the plate can only be true when the incident rays make an 

 angle with the normal to the surface. This he conceives to be 

 the fact, the particles of silver bromiodide scattered through the 

 collodion film acting to reflect the light thus obliquely. The ex- 



