86 REPORT — 1875. 



On Mirage at Sea. By Dr. J. Janssen. 



Many facts relating to the phenomena of mirage at sea are already known ; but 

 the anther has paid great attention to these appearances in aU his voyages since 

 1868, and has made some remarkable observations on mirage, especially at sunrise 

 and sunset. lie has established : — 1. That the mirage is nearly constant at the 

 sm-face of the sea. 2. That the appearances can be explained by assuming the 

 existence of a plane of total reflection, situated at a certain height above the sea. 

 3. That the phenomena are due to the thermic and hygrometric action of the sea 

 upon the neighbouring atmospheric strata. 4. That there exist at sea direct, 

 inverse, lateral, and other mirages. 6. That these phenomena have a very general 

 influence upon the apparent height of the sea-horizon, which is sometimes lowered, 

 sometimes raised. 



This variation of the apparent horizon it is very important to take into account, 

 if we consider the use made of the horizon in nautical astronomy. 



On the Photographic Revolver, and on the Observations of the Transit of Venus 

 • made in Japan. By Dr. J. Janssen, 



' ■ The author 8 expedition to Japan to observe the transit of Venus divided into 

 two parts, the one taking up its station at Nagasaki and the other at Kobi. 

 . At Nagasaki he observed the transit with an equatorial of 8 inches aper- 

 ture. 1. He obtained the two interior contacts. 2. He saw none of the pheno- 

 mena of the drop or of the ligament ; the appearances were geometrical. 3. He 

 observed facts which prove the existence of an atmosphere to Venus : he saw the 

 planet Venus before its entry on the sun's disk by the aid of suitable coloured glasses. 

 This important observation proves the existence of the coronal attnosphere. 5. 

 There was taken at Nagasaki a plate by the revolver for the first interior contact. 

 6. M. Tisserand observed the two interior contacts with a 6-inch equatorial ; the 

 contacts were sensibly geometrical. 7. Sixty photographs of the transit upon 

 silvered plates were obtained. 8. There were also obtained some photographs of 

 the tran.sit (wet collodion and albumenized glass). 



. At Kohl (weather magnificent). — Fifteen good photographs of the transit (wet 

 collodion and albumenized glass), of about 4 inches in size, were obtained : they will 

 admit of being combined with the English photographs at the southern stations. 

 The astronomical observations of the transit were made successfully byM. Delacroix, 

 who was provided with a 6-inch telescope. He observed facts which attest the 

 existence of an atmosphere round Venus. 



On a Mode of producing a sharp Meridian SJiadow. By A. Malioch. 



On the Optical Properties of a Titano-Siticic Glass. 

 By Professor Stokes and J. Hopeinson. 



At the Meeting of the Association at Edinburgh in 1871, Professor Stokes gave 

 a preliminary account of a long series of researches in which the late Mr. Vernon 

 Harcoiu-t had been engaged on the optical properties of glasses of a great variety 

 of composition, and in which, since 1802, Professor Stokes had cooperated with 

 him*. One object of the research was to obtain, if possible, two glasses which 

 should achromatize each other without leaving a secondary spectrum, or a glass 

 which should form with two others a triple combination, an objective composed 

 of which should be fiee from defects of irrationality, without requiring undue 

 curvature in the individual lenses. Among phdsphatic glasses, the series in which 

 Mr. Hareourt's experiments were for the most part carried on, the best solution of 

 this problem was oflTered by glasses in which a portion of the phosphoric was 

 replaced by titanic acid. It was found, in fact, that the substitution of titanic for 



* Report for 1871, Transactions of the Sections, p. 38. 



