434 Mr. Scrymgeour's Experiments to determine the Influence 



Newton's measures of the distances at which the first black 

 bar is formed appear to be bad. According to theory, when 

 the breadth of the aperture is very small, the distance should 

 vary as the square of the breadth. This proportion is very 

 closely preserved in Biot's experiments with very narrow aper- 

 tures, but is not at all maintained in Newton's experiments. 

 Newton's measures are therefore inconsistent with Biot's. 

 Now Biot's methods were in every respect superior to New- 

 ton's. Among the principal causes of superiority I may men- 

 tion the use of a semitransparent screen, which could be ob- 

 served behind, and the use of light almost strictly homoge- 

 neous. It is also worthy of notice that Newton's experiments 

 were not intended for publication; that Newton avers in his 

 preface, £ that he had not repeated some until he had satisfied 

 himself about all their circumstances ;' and that he informs us 

 that they 6 were put together out of scattered papers.' 



Very pressing occupations have prevented me, for the pre- 

 sent, from repeating these experiments with accurate mea- 

 sures; the only observation with measures which I have ob- 

 tained is the following : The distances from the image of the 

 sun, formed by a lens, to the aperture, and from the aperture 

 to the eye-piece, being each 60 inches, and the breadth of the 

 aperture 0*07 inch, the centre was a bar of diluted blue in- 

 closed by two bars of red-brown. This agrees well with Fres- 

 nel's theory and numbers ; for it appears on calculation that 

 the extreme blue is rather brighter at the centre than on each 

 side, and that the bright yellow is much less bright at the 

 centre than on each side. 



LXXI. Narrative of Experiments made with the Seconds Pen- 

 dulum, principally in order to determine the hitherto un- 

 assigned Amount of the Influence of certain minute Forces on 

 its llate of Motion. By Mr. James Scrymgeour. 

 [Concluded from p. 350.] 



f T , HE following experiments were made in order to deter- 

 -■• mine the nature of the effect of the proximity of any body to 

 the clock pendulum, and also to ascertain whether the current 

 of air generated by the motion of the pendulum could be re- 

 duced, or prevented from being formed. The detached pendu- 

 lum was mounted with two brass balls, weighing in all about 

 3 pounds ; the larger being 2 inches in diameter, and the other 

 somewhat less. Two pieces of deal were placed parallel to 

 the motion of the pendulum, one on each side of the balls, at 

 the distance of about J of an inch ; but the space at each end 



