optics. £43 



be cracked, nor any ways affected ; nor will the 

 water be in the least degree warmed. If the wood 

 be taken out, and the rays thrown on the water, 

 no continuance of collected rays in this way will 

 either heat the water or crack the glass ; but if a 

 piece of metal be put into the water, it soon be- 

 comes too hot to be touched, and communicating 

 its heat to the water, makes it not only warm, but 

 sometimes causes it to boil. 



Though water alone be not affected, yet, if a 

 little ink is thrown into it, the water will soon be 

 heated. 



To jind the focal Distance of Lenses by Experiment. 



1. When the focal length of the lens does not 

 exceed two or three feet, it may be found by hold- 

 ing the lens at such a distance from the wainscot, 

 opposite a window-sash, that the image of the sash 

 may be distinct upon the wainscot ; and this dis- 

 tance may be considered as the focal length of the 

 lens ; but if the focal length is long, you must 

 compute the focus by the subsequent rule. 



Measure the distance between the lens and the 

 object, and also from the image : multiply these 

 distances together, and divide the product by their 

 sum ; the quotient will give the focal distance ; 

 or, the square of the distance of the observed focus, 

 divided by the distance of the object from the 

 image, will give the excess of the observed focus 

 beyond the true focal distance. 



2. To find the focus by making a candle the 

 object. 



To do this, move the lens on the candle, and 

 the paper for receiving its image, so that when 



it 2 



