On the " Focometry of Lens-Combinations.'" 357 



vibration-forms ma}' be obtained which are identical with r 

 (a) the motion of a string bowed with suitable pressure 

 close to its centre, or at a distance from it not exceeding a 

 quarter of its length; (b) the motion of a string bowed near 

 an end with pressure insufficient to elicit the fundamental 

 pre-eminent ; and (c) the successive stages of the vibration 

 obtained by bowing a string near an end at the " wolf-note "' 

 pitch, or under certain other circumstances in which cyclical ,. 

 not periodic, vibrations are obtained. 



The photographic records are in complete accord with 

 theory, and are an interesting illustration of the experimental 

 analysis of discontinuous vibration-forms. 



The experiments described in this note were carried out in 

 the Laboratory of the Indian Association for the Cultivation 

 of Science. 



Calcutta, 

 November 8th, 1916. 



XXXIV. Note on Prof. Anderson s " Focometry of Lens- 

 Combinations." By Robert E. Baynes, M..A* 



PROF. ANDERSON'S method (p. 157) of determining 

 the focal length of a combination is most interesting, 

 but the degree of its exactness requires examination ; for the 

 data given in the paper for a particular case, viz. a , 1 =51'0, 

 y 1 = $'Q, ^2 = 21*5, y 2 =9'5, ^=26*9, lead to curious results 

 when treated in different ways. 

 Prof. Anderson's formula 



/= ^(«i/yi-**/y«) 



gives /'=8*71, but the distance between the nodal planes 

 when calculated from this value of f with x lf y x is 8' 52 and 

 with t r 2 , y 2 is 6*58, a difference which cannot be ignored. 



The fact is that there is a definite relation between the 

 five given magnitudes, viz. 



^(1 - y-&il*ix%) = (jb x —yj — (#2— y*\ 



which is not satisfied in the above case and which requires 

 d = 29'0 with the given values of a?,, . . . With this value 

 of d we should have obtained 9 '39 for the focal length and 

 5*93 for the distance between the nodal planes. 



* Communicated by the Author. 



