viii TRANSIT OF VENUS, 1874. 



employed for their treatment, I conceive it to be possible that some astro- 

 nomer may yet think them worthy of rediscussion. The photographs 

 themselves are carefully preserved at the Eoyal Observatory. I do not 

 imagine that any important improvement can be made in their measures ; 

 but perhaps - the number of photographs may be reduced by judicious 

 rejection of those whose definition is doubtful. And the theory of instru- 

 mental distortion, to which I have alluded in the Appendix to this work, may 

 be considered. 



It was recognized in some of the photographs that the limb of Venus was 

 much distorted by atmospheric action ; a corresponding distortion might be 

 expected in the Sun's limb. And it was believed by some of the measurers 

 of the photographs that irregularities of the Sun's limb, amounting to two 

 or three seconds, produced uncertainty in the measures of the photographs : 

 if that irregularity be real, it might be expected also to produce error on the 

 same scale in the optical observations of contact. 



G. B. AIRY. 



Royal Observatory, Greenwich, 

 1881, June 6. 



