Total Solar Eclipse of 1900 (May 28). 351 



5 inches within the focus, giving an image of the sun 1 inches in 

 diameter; but the " Abney " lens was no longer used in the other 

 tube. It had been decided by the Joint Permanent Committee to dis- 

 continue the separate use of the two Abney lenses, and to recombine 

 them into the original doublet, which Mr. Davidson was to use in the 

 expedition organised by the Astronomer Royal. .Hence the other half 

 of the double tube camera was set free, and it was utilised to good 

 effect by arranging hco polariscopic cameras to give images on the same 

 plate, a diagonal partition dividing the square tube into two. One 

 instrument was arranged by Mr. Xewall, and is described by him. 

 The other was similar to the apparatus used by me in India in 1898, but 

 with improvements in detail as described below. The double camera is 

 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. 



Alongside the double tube two other cameras were arranged for 

 single exposures during the greater part of totality. One was a por- 

 trait lens of 5| inches aperture and 30 inches focus, stopped down to 

 / S ; the other was a small polariscopic camera, described below. 



12. The Ccelostat. All these cameras were pointed downwards at an 

 angle of 18 with the horizon, in azimuth 42 west of south, to the 

 16-inch ccelostat used in India in 1898. The mirror of this instrument 

 was made by Dr. Common. It was silvered and sent out to Algiers 

 by the Improved Electric Glow Lamp Company, and had a very fine 



surface. The mounting and clock of the instrument were made by 

 Mr. J. Hammersley, from designs by Dr. Common. A steadier mount- 

 ing is desirable on future occasions, though the present arrangement 

 works well when there is not much wind to cause vibration. 



13. The Polarucopes. The arrangement used in India was as 

 follows : 



(A) Objective, 3i inches aperture, 18 inches focus. 



(B) Slit, of width 0'2 inch, in cardboard. 



(C) Collimator, 1| inches aperture, 6i inches focus. 



(D) Ehomb of spar, 1 inch aperture (clear). 



(E) Camera, 2 inches aperture, 9 inches focus. 



On the present occasion (E) was substituted for (A), which was of 

 inconvenient width for the space at disposal. The primary image was 

 thus reduced to half the size ; but this had the advantage that a larger 

 part of the image fell on the slit, the width of which remained the 

 same as before, being governed by the focal length of the collimator 

 and the angular separation of the images by the rhomb. The colli- 

 mator (C) and the rhomb (D) remained unchanged, but the camera 

 lens (E) was now a photographic objective of 1^ inches aperture and 

 28 inches focus, made specially by Messrs. Cooke and Sons, of York. 



