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VII. — Report on the Total Solar Eclipse of April 6, 1875. 
By J. N. Lockyer, F.R.S., and Arthur Schuster, Ph.D., F.R.A.S. 
Received June 19, 1877, and ordered to be printed in the Philosophical Transactions. 
[Plates 9-14.] 
I. INTRODUCTORY. 
It is never an easy task to make out the plan of operations for the observation of a 
phenomenon so rare and of such short duration as a total solar eclipse. We must be 
careful on the one hand not to risk failure by the adoption of new and uncertain 
methods, and on the other hand we must bear in mind that the mere repetition of 
what has been done before does not justify any large expenditure of time and money. 
In drawing out the instructions for the expedition the Committee of the Royal 
Society had to consider in how far the old methods could be perfected, and in how far 
new ones should be tried. 
The principal methods hitherto employed successfully in total solar eclipses consist 
in spectroscopic eye observations and the obtainment of photographs of the corona. 
As the Royal Society had secured the services of Professor Tacchini, and as 
Mr. Janssen intended to observe the eclipse for the French Government, it was 
considered that spectroscopic eye observations were sufficiently provided for. As far 
as the photographic impressions of the corona itself were concerned, it was arranged 
that the same instrument with which Colonel Tennant had taken his photographs 
during the Eclipse of 1871 should be sent to one of the stations with an observer 
practised in its use. 
Considering, therefore, that the attack of the corona was in excellent hands as far 
as the old methods were concerned, the Committee of the Royal Society decided to 
adopt new methods which should open fresh fields of research. 
We give a short account of what these methods were and what questions they are 
likely to solve. 
The Prismatic Camera. 
During the Total Solar Eclipse of 1871 Mr. Respighi and Mr. Lockyer inde- 
pendently made observations with a spectroscope deprived of its collimator. A series 
of rings was seen by them corresponding to the refrangibilities of the rays sent out 
by the corona. The chief object of the observers during the last eclipse was to 
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