214 SEC. 7. LIGHT. 



one or the other of the spectrum colours can be excluded, and thus its com- 

 plementary and contrast colour produced. By inserting a second axle, the 

 movements of the clockwork can be retarded. 



974. Apparatus for demonstrating the Glory on bedewed 

 Meadows, consisting of glass globes filled with water. 



Professor ]Jr. Lommel, Erlangcn. 



975. The same, with drops of Canada balsam become torpid. 



Professor Dr. Lommel, Erlangen. 



These glass globules serve to demonstrate why dew drops shone upon by the 

 sun appear so brilliant just round the shade of the head of the observer. The 

 explanation is that each dew drop is a lens concentrating the rays upon the 

 ground below the drop, which latter sends them back diffused. 



If the glass globules are placed upon a sheet of white paper, and the shadow 

 of the observer's head is permitted to fall into the circle, the globules will 

 appear very brilliant; as soon, however, as the white reflecting sheet is 

 removed, and the globules rest on a dark dull ground, the brilliancy at once 

 disappears. 



The preceding experiment can also be well executed with solidified drops of 

 Canada balm. 



976. Four Absorption Cases, all in glass, 5 and 10 mm., 

 two and two pieces for spirits and water. 



Warmbrunn, Quifitz, and Co., Berlin. 



977. Buns en's Apparatus for Experiments in Spectrum 

 Analysis, a battery of four Leydeii jars, a stand with arrange- 

 ments for producing a spark spectrum, with holder for two 

 spectrum tubes. Keiser and Schmidt, Berlin. 



977a. Actinometer. Prof. Balfour Stewart. 



978. Apparatus for converting light into chemical, electric, 

 magnetic, and mechanical motion. Length, 38 centim. ; width, 38 

 centim. ; height, 33 centim. (A drawing is sent with the instru- 

 ment.) Prof. Theodore Schwann, Liege. 



The apparatus consists of two plates of platinum placed horizontally and 

 not in contact, which serve as electrodes to a galvanometer of Du Bois Rey- 

 mond of 28,000 revolutions. The plates are enclosed in an ebony box, fitted 

 with two wooden covers, which can be taken off separately, so that one elec- 

 trode can be exposed to the light, whilst the other remains in darkness. A 

 small square piece of blotting paper is first dipped into nitrate of silver, then 

 dried, and dipped, without being exposed to light, into a solution of iodide of 

 potassium. The paper thus prepared is placed, always in darkness, on the 

 two electrodes, so that the needle of the multiplier is at zero. 



The covers being in place, light, or better still, a sun ray, is admitted out- 

 side ; one of the covers is then removed. At the end of a few seconds the 

 needle begins to deviate from 40 to 60. If the cover be replaced, the needle 

 returns to zero, and the experiment may thus be frequently repeated. 



The apparatus can also be used to determine the influence of light on 

 chemical combinations which are not photographic, or on chemical actions in 

 which light is supposed to play a part. The experiments are not as yet 

 sufficiently advanced. The apparatus is not described. 



