NOMOS. 135 



There is DO reason, therefore, why the rays of the 

 sun may not penetrate deep into the earth and be 



to be supposed that the lines of ray-currents impinging upon the lens 

 are carried through the lens, and through the district beyond the 

 lens, by provoking a continuous line of molecular changes in the 

 glass, and in the atmosphere beyond the glass. It is to be supposed 

 that these ray-currents (in obedience to the law which causes cur- 

 rents passing in the same direction to attract each other) tend to 

 attract each other. It is supposed, also, that this attraction will be 

 of unequal strength in the parts of the current which are passing 

 through the lens, and that the thickness of the lens may be taken as 

 the measure of the strength of this attraction, because there must 

 needs be more active molecules in the line where the lens is thickest. 

 Let the diagram represent a convex lens, and the lines abode 

 so many currents. After entering the lens 

 these currents converge, and they do this, 

 we think, because the currents which pass 

 towards the centre exercise a stronger 

 attraction for the currents which pass to- 

 wards the edge, than is exercised by these 

 outer currents for them. Having a power 

 which is proportionate to its length, the 



current c, that is to say, will cause the currents b and d, as well as 

 the currents a and e, to approximate towards it, while at the same 

 time the effect is increased by the movement of a and e towards b 

 and d, in consequence of the stronger attracting powers of the latter. 

 The direction of this attraction must also be such as to cause the 

 outer currents to approximate more and more towards the central 

 current, in proportion as they pass from the point at which they 

 enter the glass. It must do this because the currents are fixed in 

 their position, and not able to yield to their mutual attraction, at 

 the points where they enter the glass. This altered direction, more- 

 over, when once acquired, must continue, and thus the convergence 

 must increase more and more as we proceed from the point where it 

 commences, until it ends in a focus of heat and light, if the material 

 substance which corresponds to the focus is inadequate to convey all 

 the currents which are thus made to pass through it. Upon the 

 same principles it is also easy to understand that the position of this 

 focus must move nearer and nearer to the centre of the lens, in pro- 



K 4 



