G. WEST ROYSTON-PIGOTT ON OBJECT GLASSES. 



35 



this, that exactly at the position where the image is formed clearly 

 at the same distance from the paper as the candle is, the distance 

 between the candle and image is the least possible or a minimum. 

 A most useful result is now obtained ; in every case this minimum 

 distance is exactly four times the focal length of the lens. In the 

 case of a three-inch lens this mijiimum image distance will be found 

 to be exactly 12 inches. 



We will now suppose that instead of the three-inch lens a two- 

 inch objective is used in precisely the same way. The minimum 

 image distance between a candle and its image will be found much 

 less than 8 inches, so that the real focal length* is 1-J^f or rather 

 less than one inch and a half. 



There are two or three preliminary points which may not be 

 uninteresting. To find the focal length of a plano-convex lens, 

 turn the/«^ side to the sun, and measure the exact distance from 

 the sharpest image on a card to the convex surface. 



If the lens be equiconvex, half the thickness must be added. 



If the lens be used as a convexo-plane, and the plane side is 

 towards the image, when the aberration is reduced, two-thirds of 

 the thickness must be added. 



The minimum image-distance avoids these inconveniences of 

 measurement altogether. 



In every case the true focal length will be more accurately 

 determined by using only a small central aperture applied to the 

 lens in question. 



I have designed an instrument of considerable accuracy for 

 measuring the focal length of ordinary lenses, consisting of a per- 

 forated metal plate, and a white screen or piece of ground glass, 

 with a carrier for the object-glass or lens. By means of a long 

 screw tapped with similar right-handed and left-handed dies, the 

 perforated plate and lamp and the screen are simultaneously made 

 to approach or recede from the lens, which is thus kept always 

 exactly equidistant from the plate and from the screen. The lens 

 to be measured being fixed, the screw is turned, until an exact 

 image is formed upon the screen, of the perforations ; one- fourth 

 of the distance between them is exactly the focal length required. 

 I term this instrament a Focimeter. 



But in the case of very minute lenses, considerable difficulty is 



* Focal point of a lens is generally known to be the focus of parallel rays. 



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