352 



NATURE 



{March 4, 1875 



the prismatic camera is attached. Care should be taken 

 that the ieast refrangible part of the ring corona should 

 be recorded. The axis of the camera should cut the 

 axis of the declination axis. 



Ol'scn'iilifliis io Ic made with Tchspcctyouopcs. 



Before and after totality the cusps should be continually 

 thrown on the slit and the spectrum photographed ; long 

 exposures should be at first employed. At least one 

 spectrum of the sun should be obtained before totahty, 

 with the ordinary position of the plate, in order to indicate 

 the parts of the plate on which the various parts of the 

 spectrum falls with the angle of deviation and the orienta- 

 tion adopted. 



In all instruments just previous to totality, the vanishing 

 portion of sun is to be used to obtain a scale on the plate 

 on which the attempt will be made to obtain the spectrum 

 of Young's stratum, and the other phenomena at the 

 beginning of totality. 



For this purpose one of the end window s will be opened, 

 and all the others closed in the first in stance, the open 

 part of the slit being arranged radially over that portion 

 of the sun's light which will be the last to disappear. 

 Immediately before totality all the windows are to be 

 opened without deranging the instrument. 



The time for which the plates are to be exposed after the 

 commencement of totality will be subsequently referred to. 



For observation at end of totality all windows 

 except one at the end of sht to be opened. The part of 

 the sun which will first reappear should lie on the slit 

 just outside the closed shutter (the motion of the moon 

 being taken into account), so that the phenomena at 

 moment of reappearance may be photographed. Imme- 

 diately after reappearance the previously opened shutters 

 should be closed, and the previously closed shutter should 

 be opened to obtain the solar spectrum as a scale. Care 

 should be taken not to confound the brighter parts of the 

 chromosphere, at reappearance, with the sun itself. 



Obsemations li'ith the Pn'suuitic Camera. 

 A trial photograph can be made when i-iooth part of 

 sun's diameter is still visible. The results ol development 

 of the spectrum of the two cusps should determ.ine the 

 time of exposure before totality ; as many photographs 

 should be obtained as possible before totality, being rapidly 

 multiplied just before disappearance. The number of 

 plates to be taken during totality to be subsequently 

 referred to. The number of plates to be obtained after 

 totality will depend on results of development before 

 totality. 



2. Observations on the Polansaiion of the Corona. 



The primary object of these observations was to furnish 

 evidence on the question v/hetherthe corona was a true solar 

 phenomenon, or in some way due to a glare in the terres- 

 trial atmosphere. In the former case the position of the 

 plane of polarisation (if the light were polarised at all) 

 would have reference to the sun's centre, and would be 

 parallel or perpendicular to a line joining the centre to 

 the point observed. In other words, the polarisation would 

 be radial. In the latter case it would have reference to 

 the general direction of the observers' view ; i.e. it would 

 be uniform over the whole area of the corona. 



Former observations appear to show that the total light 

 from the corona is partly polarised ; and that the polari- 

 sation is in part radial, and in part unidirectional. In ad- 

 dition to this, spectroscopic observations have connected 

 the corona with the sun. But, although the main question 

 may consequently be considered as already settled, the 

 polariscopic observations have been found so delicate as 

 to justify their repetition. The details of polarisation, if 

 sufficiently well defined, may tell us something of the 

 condition of the matter emitting coronal light; and if to 

 former eye observations photographic pictures be added, 

 our information may be extended to regions further from 



the sun's surface than any of which we have at present 

 cognisance. 



If a Nicol's prism be placed in the tube of a telescope 

 of long focus {i.e. in which the convergence of the rays 

 from the object-glass is not so great as perceptibly to 

 affect the analysing power of the Nicol), ttieri, on turning 

 the Nicol so aS to cut off the part of the light polarised in 

 one plane, we shall see only that which is radially 

 polarised together with the unpolarised light. 



The part of the light polarised radially would, without 

 an analyser, appear as a complete ring of light, except so 

 far as it is interrupted by rifts or other irregularities ; but 

 with the Nicol the ring will appear divided into two 

 halves, brightest at the points where the radial polari- 

 sation coincides with that due to the Nicol, and shading 

 down to the intensity of the unpolarised light alone at 

 points situated 90° from the former. 



In other positions of the Nicol the atmospheric 

 polarisation will be less and less suppressed ; and at a 

 position 90° from its first, it will retain its full relative 

 intensity. 



A quartz or a biquartz might, of course, be used, but 

 with feeble light the eye is better able to distinguish 

 between differences of intensity than betv/een diffef-ences 

 of colour. 



To use the instrument sent out. On the day before the 

 eclipse, take out the eye end containing the Nicol and 

 camera and turn the Nicol, till the bottom of the camera 

 being horizontal, the light reflected at the polarising 

 angle, from a polished mahogany surface is cut off. 



The first photograph should be taken with the instru- 

 ment so adjusted, and the camera and Nicol mustbe in- 

 serted in the telescope so that the top and bottom of the 

 plate are horizontal when the telescope is directed to the 

 sun. 



The first photograph to be exposed for 25 seconds. 



Between each photograph the camera and Nicol to be 

 rotated through 30° in the direction of the hands of a 

 watch. 



It is desirable that some of the exposures should be 

 long, as by this means the extent of the corona can be 

 best determined. 



If the development shows that it may be attempted 

 with advantage, one or two photographs may be taken 

 with very short exposures. 



The adjustment of this instrument to the blue rays 

 must be most carefully determined beforehand, as the 

 object glass is not corrected for them. 



General Remarks. 

 Plates during Totality. 

 The number will depend upon experiments to be made 

 on the rapidity of drying and decrease of sensibility. If 

 it is found that plates may be exposed during the whole 

 of totality, some plates at least should be exposed for the 

 whole of that time. In prismatic camera, one may be ex- 

 posed for one minute to begin with. Whether the next 

 plate should be exposed during two or three minutes to 

 depend on results of development. 



Width of Slit. 

 Arrangements should be made for readily securing the 

 opening of slit which gives the best testing efi'ect rcfeiTed 

 to before, and a wide opening which allows at least one 

 line being seen between the H's, can be readily distin- 

 guished. This latter opening should be used in all obser- 

 vations during totality. For scale r'eterminations the first 

 position of slit should be employed. In some instruments 

 a much wider slit may be used than in others. Experi- 

 ments should be made on this point. 



Precautions to he attended to i/i preliminary experiments. 

 I. All apertures to be reduced. The slit should not 

 be exposed longer than necessary to the heating power 

 of the sun. 



