physics. 599 



in 1881 he estimates moonlight to be equal to the light of a candle 230 

 centimeters distant. The light of a cloudy sky through an aperture 

 one square inch in area was found to be equal to oue candle, and the 

 intensity of the light from the sun's disk was equal to 5,300 candles. 

 (Lond.Elec. Rev., xi, 490; Am. J. Sci., February, 1883, III, xxv, 149.) 



Crova has introduced a correction into the value of the solar luminous 

 intensity, previously given by him, and now concludes that the sun's 

 illuminating power in a clear sky is very near 8,500 carcels. Allowing 

 9.5 candles to a carcel, this gives over 80,090 candles as the value of 

 the sun's light, a number considerably greater than Sir William Thom- 

 son's estimate. (G. R., xcv, V212- xcvi, 1L'4, January, 1883.) 



McLeod has made a series of experiments to test the action of light 

 on india-rubber. He comes to the conclusion that caoutchouc alters 

 under the combined iutlucnce of light and oxygen, but either alone 

 produces no effect. (Nature, February, xxvn, 312; July, xxvni, 226.) 



Huggins has succeeded in reproducing the solar corona by photogra- 

 phy, using a solution of potassium permanganate to absorb all rays 

 different from those emitted by the corona itself. Compared with the 

 photographs obtained during the eclipse of the 17th of May, Abney 

 considers their essential identity established. (J. Phys., April, 1883, 

 II, n, 173.) 



Cros and Vergeraud have succeeded in producing direet positives by 

 the following process : Any suitable paper is covered with a solution 

 of 2 grams ammonium bichromate, 15 grams glucose, and 100 of water, 

 and dried. It is then exposed to the light under a positive. When 

 the uncovered parts of the paper have become gray it is placed in a 

 bath containing 1 gram of silver nitrate, 10 grams of acetic acid, and 

 100 grams of water. The image appears at once in red, drying to a 

 dark brown. By treatment with a sulphide it becomes black. (J. Phys., 

 March, 1883, II, n, 123.) 



Hare and Dale have constructed a multiplex camera back by which 

 thirteen plates in two tiers may be exposed, in any order without open- 

 ing the box. (Nature, September, 1883, xxvni, 470.) 



2. Reflection and Refraction. 



Lermantoff proposes to select thin disks of microscopic cover glass 

 to serve as light mirrors, by means of Newton's rings. Using a lens of 

 long focus, placed successively on the disks illuminated with sodium 

 light, those are selected which show regular rings not altered in size 

 when the disk is turned over. (Jour. Soc. Phys. Chim. Russe, xiv, 

 480; Jour. Phys., December, 1883, II, n, 5S3.) 



Laurent has communicated to the French Academy a description of 

 some new pieces of apparatus devised for the purpose of testing optical 

 surfaces. Among these are, (1) an apparatus for verifying the flatness 

 of a , plane surface; (2) one for controlling parallel surfaces; (3) one 

 for fixing the construction of prisms of any determined angle; and (4) 



