TRANSACTIONS OF THE SECTIONS. 



15 



the value of fi for the point at which the focal length is to be made a minimum ; r 

 the ratio of A ^' to A ^ to be employed in the ordinary formula for achromatism. 

 Then having calculated numerically 



we shall have 



■~''lX — fJi' '^-^ fi3- 



, 2 m - Ml — /i2 



For the value of m it will be suflBcient to take 



^D+ I ('*e-/^d)- 



On applying this formula to calculate r for the object-glass for which Fraunhofer 

 has given both the refractive indices of the component glasses and the value of r, 

 which, as observation showed, gave the best results, and taking in succession 

 various combinations of three lines each out of the seven used by Fraunhofer, the 

 author found that whenever the combination was judiciously chosen, the resulting 

 value of r was the same, whatever might have been the combination, and equal to 

 1"980, which is precisely the value determined by Fraunhofer from observation, as 

 giving the best effect. 



On a new Form of the Gas Battery. By William Symons. 

 The ingenious and original arrangement known as Grove's gas battery, although 

 always considered an instrument of great philosophical interest, appears to have 

 been little used as an instrument of research and experiment, except instudying the com- 

 binations of different gases. The author has long thought that a modification of it may 

 be usefully employed in many experiments requiring a weak but continuous current ; 

 and believes the following arrangement will be found convenient and economical. 

 Fig. 1 is a plan, and fig. 2 a section of three pairs ; the tray is made of gutta percha ; 

 it is divided into water-tight compartments about 2\ inches wide ; the length of the 

 tray will of course depend on the number of cells required, and its width on the 

 length of the strips of platinum ; its depth about 1 inch. A are small tubes to 

 keep the dilute acid at a uniform level ; B are tubes perforated through the bottom 

 of the tray, and standing above the level of the acid to admit a constant supply of 



Fiff. 1. 



hydrogen from below ; C are cells about 1 inch deep, | inch broad, and long enough 

 cover the platinum plates ; these may be composed of glass, or gutta percha with 



