NATURAL PHILOSOPHY. 203 



The requisite condition to obtain these effects is that the different 

 " coats " of wire shall be separated from each other by a piece of 

 paper, and that the interior of the bobbins, whether in wood or copper, 

 should be covered also with an isolated substance. The advantages 

 of this system are obvious, the first being reduced cost of the mag- 

 nets. Then we have greater effects, which is tantamount to a reduc- 

 tion of size, and consequently another reduction of the cost. The 

 " extra currents 11 being also done away with, a more prompt movement 

 of the armatures results, and therefore greater usefulness in induction 

 coils. In telegraphic instruments they present the additional advan- 

 tage of remaining unaffected by lightning. M. Du Monccl remarks, 

 by way of explanation explanation is easier than prediction I con- 

 sider that in magnets of the new construction the surface, of con- 

 tact of the spirals between themselves represents, in fact, a linear 

 spiral, of which the points furnish derivations. We can easily 

 imagine that the electric llux provoked by these derivations can 

 only be produced by furnishing a series of superposed currents 

 circulating through all the folds of the metallic helix, by reason 

 of the resistance to the passage from one spiral to the other. 

 Now, if the primitive current circulating through the helix is weakened 

 by these derivations, it is reinforced by the derived and superposed 

 current, which, in over-exciting the pile, furnishes at last a more 

 energetic current. Besides, it must be borne in mind that the direct 

 current which results from the derivations, and which passes through 

 the spirals towards the axis, ought to be derived from them, and as 

 it is not enfeebled by its passage, it should augment the intensity of 

 the current which flows through it." Lastly, the quantity of 

 uncovered wire which can be used for a given magnet is greater than 

 that of covered. 



ON THE INVISIBLE RAYS OF THE ELECTRIC LIGHT. 



We are so accustomed to associate the word ray with the idea of 

 light, that the term dark or obscure rays stimulates the imagination by 

 its strangeness. And such is more particularly the case when we are 

 told that the major portion of the radiation of the sun itself is of this 

 invisible character. This great discovery was announced 65 years 

 ago by Sir William Herschel. Permitting a sunbeam to pass through 

 a glass prism, he formed the colored spectrum of the solar light ; and 

 carrying a small thermometer through its various colors, he deter- 

 mined their heating power. He found this power to augment grad- 

 ually from the violet to the red ; but he also found, to ins surprise, 

 that the calorific action did not terminate where the visible spectrum 

 ended. Placing his thermometer in the dark space beyond the red, 

 he found the heating power there to be greater than in any part of 

 the visible spectrum. Sir William Herschel concluded from his exper- 

 iments that besides those rays which, acting separately upon the retina, 

 produce the sensation of color, and the sum of which constitutes our 

 ordinary sunshine, a vast outilow of perfectly invisible rays proceeds 

 from the sun ; and that, measured by the heating power, the strength 

 or energy of these invisible rays is greater than that of all the visible 

 rays taken together. 



This result was questioned by some and confirmed by others ; but, 



