CONCEUXIXC LIGHT. 85 



x. Ways and Progress of Light. 



We have attentively to consider the ways and progress 

 of light. Light is shed around on all sides ; but it remains 

 for inquiry whether it at the same time ascends a little, or 

 is equally shed around upwards and downwards. The 

 light itself generates light every where around it ; so that 

 when the body of light, on interposing a screen, is not dis 

 cerned, yet the light itself illuminates every thing around, 

 except the objects which fall within the shadow of the 

 screen : these, however, receive some light from the light 

 which is thrown around ; for any thing within the shadow 

 of the screen can be discerned much better than if no light 

 at all were present. Thus the visible body of any lucid 

 object, and the light itself, seem to be things different. 

 Light doth not penetrate bodies which are fibrous and of 

 unequal structure; but yet is not imped-ed by the solid 

 hardness of a substance, as you shall see in glass and the 

 like. Thus the straight line and the pores which are not 

 transverse, alone seem to transmit light. The best con 

 ductor of light is air, which conveys light the better the 

 purer it is. It remains for inquiry whether or not light is 

 carried through the body of the air. Sounds certainly we see 

 carried by winds, so that they may be heard farther when 

 going with the wind than against it. But it remains for 

 inquiry whether or not any thing of the kind takes place 

 with light. 



xi. Transparency of lucid Bodies. 



We must also inquire respecting the transparency of 

 lucid bodies. The wick of a candle is seen within the 

 flame; but through larger flames objects reach not the 

 sight. But again, all transparency is lost on heating any 

 body, as may be seen in glass, which is no longer trans 

 parent when heated. The substance of air is transparent, 

 also of water ; yet these two transparent substances when 

 mixed, as in snow or foam, are no longer transparent, but 

 acquire a certain light of their own. 



XIT. Cognations and Hostilities of Light. 



The cognations, and also the hostilities of light must be 

 investigated. Light, as far as regards its production, has 

 most of all cognation with three things, heat, tenuity, and 

 motion. We must, therefore, consider the marriages and 

 divorces of these three with light, also the degrees of these 

 same marriages and divorces. The flame of spirit of wine 



