optics. '245 



glass, when the circles unite being measured, gives 

 the focal distance. 



To find the focal length of a concave lens : 

 Let the lens be covered with paper, having two 

 small circular holes : and on the paper for receiv- 

 ing the light, describe also two small circles, but 

 with the centres at twice the distance from each 

 other of the centres of the circles. Then move 

 the paper forwards and backwards, till the middle 

 of the sun's light, coming through the holes, falls 

 exactly on the middle of the circles : that distance 

 of the paper from the lens will be the focal length 

 required. 



To find the focus of a plano-convex, and a plano- 

 concave lens : 



By similar experiments you will find, 1. That 

 the focus of a plano-convex, or of a plano-concave 

 glass, is equal to a diameter of its convex or con- 

 cave surface, that is, of the whole sphere it be- 

 longs to. 2. That the focal distance of a double con- 

 vex, or double concave glass of equal convexities or 

 concavities, is equal to a semi-diameter of either of 

 its surfaces ; and, consequently, that the focal dis- 

 tance of a glass, of unequal convexities or conca- 

 vities, will have an intermediate length between a 

 diameter and a semi- diameter, of that surface which 

 is most convex or concave. 



To measure the focal distance of a globe of 

 water, and of glass: 



Take a hollow globe of glass, or, instead of it, 

 a thin round flask, or decanter, and making a round 

 hole, about an inch diameter, in a piece of brown 

 paper, paste it on one side of the body of the de- 

 canter ; and having filled it with water, hold the 

 covered side to the sun, that the perpendicular 



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