Updegraff — Flexure of Telescopes. 265 



effect arising from displacement of the neutral axis by action 

 of the compressive and tensile forces upon the two halves of 

 the tube when in an inclined position may, on account of non- 

 homogeneity of material, differ for supplementary zenith 

 distances. 



That these causes and their effects must exist is suflSciently 

 clear. The question is whether the effects are likely to be 

 appreciable. 



The R — D discordance found many years ago to exist in case 

 of the Greenwich Transit Circle was considered by Sir G. B. 

 Airy to be due to refraction of the light in passing from the cool 

 air outside to the warmer air surrounding the mercury basin.* 

 More recent researches at Greenwich seem to show that the 

 discrepancy is due to instrumental error. f A similar dis- 

 crepancy has been found in observations made with the transit 

 circle at the Cape of Good Hope. 



After correcting the observations for sine flexure by the 

 formula h = l".3sinZ.Z>. the results for R — Z> at Greenwich 

 as given by Professor Turner in Mon. Not. R. A. /S\, Vol. 

 LIV., p. 487, are as follows: — 



On account of mechanical difficulties, observations by reflec- 

 tion are not made at Greenwich outside of the above limits 

 of zenith distance. 



The results for R — jD at the Cape of Good Hope after 

 being corrected for sine flexure by the formula h = — 0".46 

 sinZ.Z). are as follows :| — 



* Memoirs R. A. S., Vol. XXXII., pp. 9-17. 



t Mon. Not. B. A. S., Vol. LIV., p. 486. 



t Introduction to Cape Catalogue of Stars for 1885, p. XVII. 



