24fi optics. 



rays may pass through the middle of the water, 

 and the emergent rays will be collected to a focus, 

 whose nearest distance from the decanter will be 

 equal to the semi-diameter of the body of it, as 

 will appear by receiving the rays upon a paper held 

 at that distance. That this effect is owing to the 

 water, and not to the glass, will be evident by 

 emptying the decanter ; for the light that then 

 passes through the hole, will be as broad as the 

 hole itself, at all the distances of the paper from 

 the decanter. 



If a similar experiment be tried with a solid 

 globe or ball of glass, the distance of the focus 

 from the nearest part of the ball will be one 

 quarter of its diameter. 



To find the vertex or centre of a lens : 



Hold the lens at a proper distance from the eye, 

 and observe the two reflected images of a candle 

 made by the two surfaces. Move the lens till 

 these images coincide, and that point is the vertex; 

 and if this be in the middle of its surface, the glass 

 is truly centered. 



Whatever be the shape and magnitude of the 

 hole in the paper that covers part of a lens, the 

 shape and magnitude of the image will be the same 

 as when the lens is uncovered ; because any small 

 part of a pencil of rays has the same focus as the 

 whole ; but the brightness will be diminished, in 

 proportion as the hole in the cover is diminished j 

 because the quantity of light which illuminates 

 every point of a picture is diminished in that pro- 

 portion. 



