123 Chronicles of Science. [Jan., 



tion) to receive the illumined picture. The axis of the objective must 

 be perpendicular to it, and also out of the angle of reflection, the 

 object-glass refracting the image upon the screen. 



Heat. — By far the most valuable series of memoirs relating to the 

 science of heat, have been lately published by Professor Tyndall. It 

 would be out of our power to give, even were we to largely exceed the 

 space at our disposal, an abstract of these important researches. We 

 may, however, state briefly some of the results at which this inde- 

 fatigable experimentalist has arrived. The researches have been 

 mainly directed to the obscure, extra red rays of the spectrum ; and as 

 rock salt is the only known solid transparent substance which allows 

 all of tbese dark heat rays to pass through with equal facility, the 

 research has been delayed in its earlier stages by the difficulty of 

 getting clear pieces of this mineral of a sufficient size. ' This difficulty 

 is, happily, now overcome, and Professor Tyndall has lately had made 

 for him a complete rock-salt train of a size sufficient to permit of its 

 baing substituted for the ordinary glass train of a Duboscq's electric lamp. 

 A double rock-salt lens placed in the camera renders the rays parallel ; 

 the parallel rays then pass through a slit, and a second rock-salt lens 

 placed without the camera produces at an appropriate distance an image 

 of the slit. Behind this lens is placed a rock-salt prism, and a thermo- 

 electric pile is employed to examine the spectrum produced by the 

 prism. With both gas and hydrogen flames the maximum of heat is 

 obtained, when the pile is placed outside the visible spectrum, a little 

 beyond the red end. This was to be anticipated from the experiments 

 of Sir William and Sir John Herschel. When a spiral of platinum wire 

 is introduced into the hydrogen flame, the radiation of heat is increased 

 from 33° to 52° as measured by the galvanometer. The action of heat 

 on the pile was still very sensible when it was removed away from the 

 red end of the spectrum as far as that is from the violet, thus proving 

 that the heat spectrum is at least as long as the light spectrum. Upon 

 substituting for the flame a coil of platinum wire, which could be 

 rendered incandescent by a small galvanic battery, and placing the 

 thermo-electric pile in the position of maximum heat in the spectrum, 

 the intensity of the current which ignited the spiral was gradually 

 increased from darkness to a full white heat. The deflection of the 

 galvanometer, which at the dark heat was 1°, rose to 18° at a red heat; 

 44 - 4 at a bright red heat, and at a full white heat it was 60.° It will 

 be observed that as the refrangibility of these heat rays remained un- 

 altered, their increased intensity was due to greater amplitude of the 

 vibrations. Allusion has been made on a former occasion to the 

 transparency of an optically opaque solution of iodine to the ultra-red 

 rays. More accurate experiments have shown that with intense 

 sources of heat this liquid loses some of its transparency. Thus, the 

 whole of the heat rays from a dark spiral, a red-hot spiral, and a 

 hydrogen flame pass through the black solution of iodine in bisul- 

 phide of carbon, whilst from a gas flame only 96 per cent pass, and 

 from the electric light only 90. Dr. Tyndall has found that the 

 actual proportion of luminous to obscure rays emitted from white-hot 



