518 . Transactions of the Societ;/. 



Now I have for many years past used in my laboratory a 

 method of determining focal lengths based directly upon this 

 definition of the power of a lens. A plan of the apparatus is 

 shown in Fig. 55. 



A scale with two vertical lines upon it, at a distance h apart, 

 is mounted at a distance A away from a telecentric disk D, placed 

 in the principal focal plane of the lens L to be tested. This lens 

 projects an image h\ of the scale h, which image is again projected 

 by the objective of a Microscope as a length h 2 on to an eye-piece 

 scale. Let this Microscope have an initial magnifying power M. 



rp, . h x f , hi 1 



llien, since — = ~ and -=■ = — 



h A h 2 M 



h- = I 

 hM. " : A 



That is, f = — =-=? = Jch 2 



where k is a constant of the instrument, to which we can, by suit- 

 able designing of the optical elements, give any value we please. 



Let *=m= 10 



and let M = 2 • 5 



Then A = 10 X 2-5 = 25 



h 



If then A is made equal to a metre, h must be made equal to 

 4 cm. If with these dimensions of the elements of the system, the 

 eye-piece of the observing Microscope be fitted with a scale 1 cm. 

 long, divided into one hundred parts, then the fecal length of any 

 lens being tested can be read off directly, by reading each scale- 

 division as 1 mm. 



Rotating -table Focometer. — When a well-graduated circular 

 table, rotating about a vertical axis, such as that found on a theo- 

 dolite or spectroscope, is available, it can be employed as a focometer 

 giving excellent results. A milimetre scale, say, should be fixed 

 very accurately in the principal focal plane of the lens to be tested, 

 and the two then secured in some way on the table. The latter 

 should then be rotated until some distant vertical object, a flagstaff 

 it may be, is focused on a central scale division of the millimetre 

 scale. The azimuth reading of the table should then be taken, and 

 the table again rotated until the image of the distant object is 

 coincident with the next scale division. By taking a second 



