1870.] The approaching Total Solar Eclipse. 481 



Tennant have failed through such a course ? He says, " thinking 

 that want of light prevented my seeing the bright lines which I 

 had fully expected to see on the lower strata of the corona, I opened 

 the jaws of the slit." It is worth noticing that failure may arise 

 from this very adjustment. Too narrow a slit is clearly unfavour- 

 able, because a certain quantity of light is required for distinct 

 vision ; but on the other hand too wide a slit is equally unfavour- 

 able, because a certain relative superiority in the brightness of the 

 lines (or in this case bands) over the background of the continuous 

 spectrum is equally requisite. The obvious conclusion is, that a 

 telescope of large aperture and therefore of high light-gathering 

 power should be employed, and the light of the continuous back- 

 ground reduced as much as possible by increasing the dispersion. 



As respects the polariscopic operations, there is a similar con- 

 tradiction to be explained during the approaching observations. 

 The observers of the Indian eclipse assert positively that the light 

 of the corona is polarized in a plane through the sun's centre ; the 

 American observers, on the other hand, as positively deny this. 

 The Astronomer Koyal (than whom no higher authority — on this 

 particular subject — exists) solves the difficulty summarily by ex- 

 pressing his belief that the observers in India were not sufficiently 

 familiar with the principles of polariscopic research to interpret 

 what they saw. In this case, and assuming a similar state of 

 things in the case of the American observers, we must look forward 

 to the approaching eclipse as likely to supply the first really reli- 

 able information yet obtained respecting the polarization of the 

 corona. We cannot doubt that the observers next December will 

 not fail from want of knowledge, since not only has the Astronomer 

 Koyal called special attention to the necessity of their carefully pre- 

 paring themselves beforehand, but the government of the party has 

 been assigned to Professor Pritchard, who is nothing if not a master 

 of the science of theoretical optics. Our great fear is, however, lest 

 the methods of testing light at present in vogue may not be suffi- 

 ciently effective for the resolution of the somewhat difficult pro- 

 blems depending on the polarization of the corona. Whatever 

 advantage there may ordinarily be in the use of well-tried methods, 

 it may be questioned whether in this particular case more powerful 

 instruments than the polariscopes at present in use might not be 

 devised and employed with advantage. 



We pass over the photographic department of the expedition ; 

 in the first place because there is every reason to feel confidence 

 that under the able supervision of Messrs. Browning and Brothers 

 (at Gibraltar and Syracuse respectively) the photographic arrange- 

 ments will be exceptionally successful, and in the second because as 

 regards the inquiry, which is the main purpose of the expedition, 

 photography can teach us comparatively little. Unless the whole 



