1 36 Prof. Forbes ofi the Refractiori and Polarization of Heat. 



which, with practice, ^th of a degree may always be observed, 

 and even minuter quantities occasionally estimated. These 

 divisions are not necessarily proportional to the intensities of 

 the currents which produce the corresponding deviations. 

 The coils of wire, extending a long way on each side of zero, 

 prevent the effect from diminishing so rapidly as if they were 

 concentrated there; and M. Melloni has described, in his 

 paper in the Annates de Chimie for May 1833, a simple and 

 satisfactory method of estimating the relative values of de- 

 grees, at different points of the scale. He states, however, 

 that, under 20° of deviation, he found them quite uniform. 

 In the following experiments, the deviations were generally 

 under 15°, and in almost no case exceeded 20°. I have there- 

 fore assumed the forces to be as the deviations. Besides, no 

 change of importance would take place from a deviation from 

 this law by a small quantity. 



5. It will be perceived in the experiments which are to be 

 detailed, that the determination of all the more important facts 

 depend generally on whether one effect be greater or less 

 than another, without much regard to their absolute amount. 

 Now, the confidence which we can place in the uniformity of 

 this instrument, or at least of the small changes capable of 

 affecting it (since it is not liable like thermometers, and espe- 

 cially air-thermometers, to advance by starts,) is such, as to 

 admit of almost indefinite subdivision, where the relations of 

 small quantities are alone concerned. I conceived, therefore, 

 that without impairing its sensibility by lengthening the gal- 

 vanometer needle, we might advantageously magnify the divi- 

 sions by optical means. This I proposed to do by observing 

 the motions of the index by means of a small telescope, fixing 

 in front of the object glass a lens whose focus is situated at 

 the part of the scale desired to be magnified. It might also 

 be easy, in order to compare larger quantities, to make this mi- 

 crometrical system revolve so as to be always similarly placed 

 as regards the needle, and thus avoid the effects of parallax, 

 which at present require constant vigilance. 



6. The method here indicated, I have put into practice 

 with the greatest success in my later researches ; one tenth of 

 a degree becomes easily visible, and the constancy of the in- 

 dications fully justifies this method of microscopic examination, 

 which has enabled me to verify the most delicate deductions I 

 had drawn from simple observation, and to obtain results which 

 otherwise I must have been unable confidently to announce. 



7. For the precautions to be employed in the use of the 

 thermo-multiplier, I must refer to the first of M. Melloni's 

 very original papers in the Annates de Chimie (for May 1833), 



