TRANSACTIONS OF THE SECTIONS. 5 



The second method of dealing with optical distortion aims at total exclusion of 

 this source of error. It has been proposed by jVmerican astronomers, who intend 

 taking part in the coming observations of the transit of Venus, to exclude the 

 secondary maguitier, and, in order to obtain an image of sufficient diameter, to 

 employ a lens of considerable focal length, say 40 feet, which would give an image 

 as large as with the Kew photoheliograph — namely, 4 inches in diameter. As it 

 would be inconvenient to mount such an instrument equatorially, it is proposed 

 to fix it in the meridian in a horizontal position, and reflect the sun in the 

 direction of its axis by means of a flat mirror moved by a heliostat. There can- 

 not be any doubt about the fact that the image so produced would be nearly 

 free from optical distortion, if the intei-posed mirror did not introduce a new 

 som-ce of error. The difficulty of producing a plane mirror is well known ; and 

 there is a difficulty in maintaining its true figure in all positions ; there is also 

 a liability of the disturbance of the rays by currents of heated air between 

 the mirror and object-glass : moreover, with such an instrument position- wires 

 could not be defined with sharpness on the photographs. On the whole, greater 

 reliance may be placed on a method which admits the existence of a distortmg in- 

 fluence, but has at the same time means of checking and controlling it numerically. 



Great attention has been paid by me at various times to those efl'ects of distortion 

 which might arise from the process of drying. The residts to which the experi- 

 ments lead seem to prove that there is no appreciable contraction except in 

 thickness, and that the collodion film does not become distorted, provided the 

 rims of the glass plates have been well ground : this point is a fundamental one. 

 But in such observations as that of the transit of Venus, no refinement of cor- 

 rection ought to be neglected ; hence fresh experiments will be undertaken 

 to set at rest the question whether distortion of the film really takes place 

 when proper precautions are taken. This will be done both by the method I 

 have emploj'eil before, and also in accordance with M. Paschen's proposal to measure 

 images of such reticules as above described : tbis reticule might, as he has sug- 

 gested, be photographed during the transit of Venus, so that each plate would thus 

 bear data for the correction due to unequal shrinkage, if such were to take place. 



It has been objected by some astronomers who have casually examined solar 

 photograms that the limb of the sun appears, as a consequence of the gradual 

 shading oft", even under a small magnifying-power, not bounded by a sharp 

 contour ; but the measurements of such photograms which have been made during 

 the last ten years, of pictures taken under the most varying conditions which 

 influence definition, have proved that even the worst picture leads to a very 

 satisfactoiy determination of the sun's semidiameter and centre ; moreover an 

 independent examination of this question by M. Paschen gave as the result that 

 the mean error of a determination is only + O'OOS millimetre with a sun-picture of 

 4 Paris inches in diameter ; this con'espouds to + 0"'1.35, and it is nearly three times 

 less than that resulting from a measurement with the Konigsberg heliometer. 



Nevertheless it will be seen from the foregoing remarks that I have not hesitated 

 to arouse your attention to the fact that Astronomical Photography is about to 

 be put to the severest test possible in dealing with such a fundamental problem of 

 astronomy as the determination of the sun's distance from the earth. An intimate 

 knowledge of the subject, however, and experience with respect to work already 

 accomplished in the Kew ten-year solar observations, inspu-e me with a confident 

 anticipation that it will prove fully equal to the occasion. 



So much for performances to be looked forward to in the future : now let me 

 briefly review what Astronomical Photogi'aphy has already undoubtedly accom- 

 plished. 



In the first instance the possibility proved of giving to the photogTaphic method 

 of observation a trustworthiness which direct observations can never qiute obtain, 

 will render the results of our discussion of the ten years' solar obsei-vations at Kew 

 more free from doubts than those observational series on the Sun's spots which 

 have preceded ours. The evidence of a probable connexion between planetary posi- 

 tions and solar activity, and the evidence which we have published on the nature 

 of spots as depressions of solar matter, could never have been brought forward but 

 for the preservation of true records of the phenomena through a number of years ; 



