OP ARTS AND SCIENCES. 



367 



Percentage of radiation transmitted — 



by distilled water =15.3 



„ 10% alum solution = 13.2 



The following galvanometer readings for "lamp radiation" (as just 

 described) are given in full to show the method of procedure, and also 

 the liability to variation in successive exposures when great instru- 

 mental sensitiveness is not required : — 



This is given as a fair sample of the average error on such work as 

 is involved in most of Melloni's experiments ; where great sensitive- 

 ness in the galvanometer (according to the modern standard of what 

 constitutes sensitiveness) is not required. 



The first measures, on nearly homogeneous rays in the diffraction 

 (reflection) spectrum, ever taken by any one that I know of, were 

 taken by this instrument on Oct. 7, 1880, used with an extremely 

 delicate reiiecting galvanometer by Elliot, of about 20 ohms resistance 

 and a reflecting grating on speculum metal by Mr. Rutherford of 681 

 lines to the millimetre. Measures have been taken every fair day since, 

 the source of energy being the sim. 



The rays from a slit five metres from the grating fell directly on it 

 (without any collimator); these were, after diffraction, received on a 

 silvered glass mirror, and this formed an image of the first spectrum 

 about 20 centimetres long (from A. ==: .0004 to A, = .0007) and 8 mm. 

 wide. The " Balance " then, whose acting face is only about ^-^ the 

 length of the visible spectrum, and less than ■^\^ the length within 

 which energy is found in a degree sufficient for it to measure, receives 

 nearly homogeneous rays (which have passed through no absorbing 

 medium whatever except the solar and terrestrial atmospheres), and 

 this extremely minute amount of heat is found to give a galvanometer 

 deflection of some hundred divisions, where thermopiles have hitherto 

 failed to register any (on homogeneous rays). 



The corrections for minute selective absorption in speculum metal 

 and silver reflections, and in the metal of Balance strips, are still to be 



