10 Prof. H. H. Turner. 



PART III. SEPARATE REPORT BY PROFESSOR TURNER. 



Instrumental Equipment. 



1. The Camera. The double camera used in the Fundium Expe- 

 dition of 1893 bj Sergeant Kearney, and taken out to Japan in 1896 

 without result. The tube is of wood, 6 feet long and 14 X 7 inches 

 in section, divided by a partition into two tubes of 7 X 7 inches 

 section. In one of these is placed the " Abney "* lens of 4-inch 

 aperture and 62 -inch focal length, giving an image of the sun 

 O57 inch in diameter; in the other the photoheliograph objective 

 No. 2 (used in Transit of Venus expeditions), of 4-inch aperture and 

 5-feet focal length, with a Dallmeyer secondary magnifier of 7-J inches 

 focus placed 5 inches within the focus, and giving an image of the 

 sun 1^ inches in diameter ; the camera furnished with six plate- 

 holders, each taking two plates of 160 x 160 mm. (as in use for the 

 Astrographic Chart), both plates being exposed by a quarter- turn of 

 one shutter. 



2. The Ccelostat. The camera was pointed to a 16 -inch coelostat, 

 the mirror of which was made by Dr. Common, the mounting and 

 clock by Mr. J. Hammersley from designs by Dr. Common. 



3. The Polariscope. On the tube, and pointing to the same 

 coelostat, was a polariscopic apparatus consisting of an ordinary slit 

 spectroscope (and telescope), with an Iceland spar double image 

 rhomb substituted for the ordinary prisms. The instrument in its 

 normal state had been used by Mr. Maunder in the 1886 eclipse 

 expedition for photographing the spectrum of the corona. It is 

 fitted with two plate-holders. The dimensions are as follow : 



Objective 3J inches aperture, 18 inches focal length. 



Collimator 1J 6^ 



Camera 2 9 



The use of the apparatus will be understood from the analogy of 

 the ordinary spectroscope, regarding the Iceland spar rhomb as a 

 prism, giving only two colours the two kinds of polarised light. 

 We can thus use a wide slit, opening it until the two images of the 

 slit are just in contact without overlap. This was found to happen 

 with the apparatus in use with a slit of width 0*19 inch. The 



* One -half of a doublet photographic lens by Dallmeyer, belonging to Captain 

 Abney, used in the Eclipse Expeditions of 1886, 1887, 1889, 1893, and 1896, 

 acquired from him by the Boyal Astronomical Society in 1893 for permanent use 

 in eclipse expeditions, in consideration of their replacement by two others. 



