Page:Proceedings of the Royal Society of London Vol 69.djvu/261

Rh Abney squares were printed on No. 6 by exposure to a standard candle at a distance of 5 feet for 4 minutes with the Dallmeyer lens, and 8 minutes with the Unar lens. It had been intended to print squares on several plates, but the number of photographs to be dealt with made it necessary to limit work as much as possible the night before the eclipse when the photographic plates were taken from sealed tins and put in their carriers, and after the eclipse it was necessary to finish the developing as soon as possible. Of these photographs Nos. 5, 6, 7, 8 are the most successful. The extension shown with the Abney lens reaches to 66' from the centre, and with the Unar lens to 77'.

Venus and Mercury and a number of stars are shown on the Unar plates. No. 5 is the best, and shows 7 stars of the Pleiades and 25 others of magnitudes ranging from 4 to 6, which have been identified. Unfortunately, owing to the clouds, stars are not equally well shown all over the plate, and S Tauri (4 m> 0) and Tauri (3 m> 9) are not shown ; y Tauri (3 m> 9), which is near 6 Tauri, is, however, shown.

It may be concluded that any object appreciably brighter than 4 m- within the area covered by the plate (i.e. within 15 of the Sun) would have been shown.

III. The Spedroscopic Cameras.

The two spectroscopes used by Captain Hills at the Indian Eclipse of 1898 were kindly lent by him, and were used in Sumatra with the following adjustments :

Objective

Collimator and ea- rner* lenses Prisms.

Slit

Priams at mnxi-

mum deviation

for

Spectroscope No. 1.

Cooke, achromatic, 4^-in.

aperture, 6-ft. 2^-in. focus. Single quartz lens, 2^-in.

nperture, 30-in. focus. Two dense flint prisms of

<>0, 4i-in. base, 2Hn.

height.

H in. by 0'0015 in. H (A 4340)

Spectroscope No. 2.

Single quartz lens, 5-in. aperture, 4-ft. 7-in. focus.

Single quartz lens, 3-in. aperture, 33$ -in. focus.

Four double quartz prisVns of 60 (each prism being composed of two half- prisms of riglit- and left- handed quartz), 3-in. base, 2-in. height.

2 in. by 0'0012 in.

X3500.

The width of the slit of the flint spectroscope was obtained by means of the diffraction images, and that of the quartz was checked in the same manner.

The length of the spectrum of the flint is 3^ inches from H/s