691 



drawn-out point, which passes through tiie ahoit tube in (he upper 

 wall of the bronze cylinder, witii this short tube was effected b}' means 

 of an airpump (ub(;anda little collodion. A Gaedk mercury airpump 

 exhausts the space. The compensator-adjustment was 56.84, hence 

 exactly the sauje as that which prevails for merenry without adhering 

 layer of air, accoiding lo the observations communicated in this §. 



11. Inquiry into a diference In the optical constants of liquid and 

 solid mercury. At first the air in the bronze cylinder was very 

 carefully dried. The cooling of (he cylinder and the dish with 

 mercury in it took place by putting a mixture of solid carbonic acid 

 and ether on a tin plate screwed on to the bottom of the cylindrical 

 case. By means of ebonite as heat-insulator the conduction of heat from 

 the metallic parts of the goniometer to the cylinder is prevented. In 

 order to prevent (he cooling of the glass windows in the side tubes, 

 because no water vapour from the air may settle on them, these 

 windows are cemented to ebonite tubes, which are screwed on to 

 the side tubes of the bronze cylinder. The air-tight closure was 

 effected by means of very tough Ramsay -grease. As it appeared that in 

 spite of (his precaution some water-vapour deposits on the glasses, a 

 current of dry air was blown along them by means of a Gaede 

 box pump, which quite remedied this evil. 



During the cooling the following phenomena are observed. When 

 during the cooling the compensator is adjusted for the dark compen- 

 sator-fringe, this winds, while the mercury is still liquid and becomes 

 less dark, after which it disappears altogether. At last the fringe, 

 which has then become black again, jumps back to its original 

 position. When the telescope, which is placed behind the analyzer, 

 is then adjusted on the mercury, ice crystals appear to float on the 

 mercury. From this it is evident, that the explanation of the observed 

 phenomenon is the following. In spite of (he careful drying the air 

 contains traces of water vapour, which are deposited on the mercury 

 surface during the cooling and spread over it as a liquid ^). The 

 compensator-fringe is probably sinuous, when the water layer 

 consists of incpherent patches and is very thin. When (he water 

 forms a coherent layer and this layer has become so thick, 

 that the reflection takes place on water, the fringe disappears, as at 

 the chosen angle of incidence of 79° 46' water does not perceptibly 

 polarize the light elliptically on reflection. As soon as the tempera- 



1) These phenomena are not observed on an iron mirror during the cooling, 

 so that the condensed water vapour does not seem lo spread over this mirror. 



45* 



