THE TOTAL SOLAR ECLIPSE OF APRIL 6, 1875. 
145 
the disappointment of the observers, it only brought a letter from Sir William 
Wiseman, Commander of the ‘ Lapwing,’ announcing that he had arrived at Bangkok, 
and asking in what way he could assist the expedition. Owing to the great readiness 
with which the Siamese facilitated communication with the capital, the long-expected 
help from the ‘ Lapwing ’ arrived at last on Saturday night, the 3rd of April. During 
the two remaining days those of the officers and crew which Sir William Wiseman 
had sent did everything in their power to assist the expedition, and contributed not a 
little to the successful completion of the preparations. 
The erection of the siderostat gave comparatively little trouble ; owing to the 
excellent packing of Messrs. Cooke, it was lifted out of the case almost ready for use. 
A brick foundation had been built for it, and the adjustments were made in the usual 
way. The beam of rays reflected from the mirror were thrown into a telescope lent to 
the expedition by Mr. Lockyer. The telescope was placed inside a hut so as to 
protect it from showers of rain. A movable cover could be placed over the siderostat. 
The image given by the telescope was focussed on the slit of a spectroscope which was 
provided with quartz lenses and prisms. 
We cannot help mentioning here the great advantages which the siderostat 
possesses over the equatorial in temporary observatories. It is always comparatively 
easy to find a sure foundation for the plate on which the siderostat rests, while it is 
much more difficult firmly to fix the pillar of the equatorial into the ground. The 
horizontal telescope belonging to the siderostat can be easily levelled and firmly fixed 
with brick and mortar. In latitudes, moreover, where the sun stands high at noon, 
solar observations with a refracting equatorial are very inconvenient. 
The steamer which had carried the officers of the ‘ Lapwing ’ also had brought 
Mr. Eschke, assistant to Mr. Vogel, who had been on one of the expeditions sent out 
to observe the transit of Venus, and had come to Bangkok in the hope of finding 
Professor H. W. Vogel, who, as we have already mentioned, was attached to that part 
of the expedition which had proceeded to the Nicobar Islands. As, however, the 
expedition was greatly in want of another gentleman skilled in photography his arrival 
proved fortunate. The coating and development of the plates could now be altogether 
separated and carried on in two separate rooms, which helped to prevent confusion 
during the eclipse. The spectroscopes were adjusted in the usual way. Photographs 
of distant objects were taken, and when the camera had been in this way adjusted for 
almost parallel rays the prisms were set into minimum deviation for the violet rays, 
and the collimators adjusted so as to send parallel rays through the prisms. One of 
the collimators was found to be too long for accurate adjustment. As it could not be 
shortened easily, and as the wooden camera was easily made shorter, the latter plan 
was preferred to bring out the Fraunhofer lines. We confess that this was an 
unfortunate mistake, as converging rays passed in this way through the prism. It 
must, however, be remembered that part of the day only preceding the eclipse could 
be entirely given up to the adjustment of the spectroscopes, and that owing to the 
MDCCCLXXVIII. U 
