62 



Mr. Rollo Appleyard on a 



bank ; (2) join the two points thus obtained ; and (3) repeat 

 these operations for another pair of values of u and v. Then 

 the coordinates of the intersection of the two lines give the 

 focal length required. An additional check is obtained by 

 taking a third pair of values of u and v, or by drawing through 

 the origin of coordinates a line inclined at an angle of 45° 

 with the axes. In either case the three lines obtained should 

 intersect in a single point. In any actual experiment this is 

 not likely to be precisely the case ; but the value of / can 

 readily be inferred, or, in case of a great discrepancy, the 

 results rejected and the experiment repeated. Below are the 

 data for determining the focal lengths of a concave mirror 

 and a convex lens, which are utilized in the figure. 



Example I. — Concave Mirror. 





Observed values of u. 



Observed values of v. 



First pair 



cm. 

 60 



33 



41 



cm. 

 29-7 



50 



39 



Second pair 



Check pair 



Eesult:— f=20 centira. nearly. 



Example II. — Convex 



Lens. 





Observed values of u. 



Observed values of v. 



First pair 



cm. 

 30-5 



15 



cm. 

 -19 



-61 



Second pair 



Result :— /== — 12 centim. nearly. 



V. A " Direct-reading " Platinum Thermometer, 

 By Rollo Appleyard*. 



MESSRS. Callendar and Griffiths and their colleagues 

 at Cambridge have proved that the platinum thermo- 

 meter is capable of measuring temperature with an accuracy 

 of one thousandth of a Centigrade degree. For general 



* Communicated by the Physical Society : read November 22, 1895. 



