1871.] 



Heat. 277 



cient. A small drop of this fluid is preferred by some to arrange their diatoms 

 in instead of glycerine. 



Mr. F. H. Wenham contributes a valuable paper on " Object-Glasses and their 

 Definition," to the January number of the " Monthly Microscopical Journal." 

 It is an admirable sequel to his series of papers on the construction of object- 

 glasses, vol. i., p. in, &c, of the same journal. After criticising the papers of 

 Dr. Pigott,* and calling attention to the fact of the reduction of angular 

 aperture of the objective, when used on objects mounted in Canada balsam, 

 demonstrated by himself and Professor Robinson in 1854,! ne proceeds to explain, 

 by the aid of a diagram on a large scale, the course of the rays through an 

 eighth objective of 130 aperture of his own construction, and giving reasons for 

 the various combinations of lenses employed in it. Some remarks are made 

 respecting the vexed question of the structure of the Podura scale, and the 

 appearances shown in Colonel Dr. Woodward's photographs defended against 

 Dr. Pigott. Mr. Wenham bears testimony to the extreme value of the 

 " mercury globule " as a test for ascertaining the nature of the aberrations of 

 object-glasses during their construction, and gives some details of the mode of 

 reading its delicate indications. The subject of immersion lenses is treated in 

 a very plain manner, and their effects illustrated by large diagrams. Of the one 

 described, he writes as follows : — " The effect of this immersion lens is to give 

 greater clearness and brilliancy to the object, and render markings more distinct 

 that were before scarcely visible with the dry lens. This is, in fact, attribut- 

 able to the saving of light and comparative absence of refraction and reflection 

 from the top surface of cover and front of lens ; but the great merit consists in 

 the perfect correction that the adjustable thickness of the water stratum affords 

 in compensating for every thickness of cover. Nor is the thickness of an 

 immersion front a matter of particular nicety, for it can be made as thin as 

 desirable ; the water will occupy the place of the deficiency." The whole 

 paper is one of great interest at the present time, when the subject of high 

 power definition is being actively discussed. 



HEAT. 



The following criticism of a paper by the Rev. H. Highton has been for- 

 warded to the Editor by John Hopkinson, D.Sc. : — Mr. Highton's first chapter 

 is devoted to the consideration of the heat produced in the various parts of 

 a conducting circuit by the passage of an electric current. The paradoxes 

 there proposed seem to be founded on a misconception as to what the received 

 views on the subject are. He assumes that the heat generated in a voltameter, 

 or battery cell, follows the same law as in a metal wire — that it is proportional 

 to the resistance and square of the intensity, and is independent of the chemical 

 decompositions which may take place; from this, by reasoning which involves 

 further assumption, he shows that the energy produced by a given consumption 

 of zinc is variable. To show that he is mistaken about the facts, I will quote 

 from Jamin's " Cours de Physique," vol. iii., p. 173: — " M. Becquerel admits 

 that it (Joule's relation between heat and intensity of current) is applicable 

 when we eledtrolyse sulphate of copper with copper electrodes, because, though 

 on the one hand, there is solution of metal, and consequently absorption of 

 heat at the positive pole, on the other there is*a deposit of an equal quantity 

 of copper and a disengagement of the same quantity of heat at the negative 

 pole. Everything goes on as if there were no decomposition, and the law, 

 heat = Kn' s , applies to this liquid as to a solid. The circumstances are totally 

 different when the voltameter considered contains water with platinum 

 electrodes. The gases, as they are disengaged, absorb all the heat which they 

 produce in combining ; it is proportional to their quantity ; that is, to the 

 intensity, i, and a constant, E. Consequently, the heat found in the voltameter 

 ous;ht to be equal to Kri* — Ei. Experiment confirms this explanation." It 

 will be seen, then, that the fundamental assumptions of the first chapter are 

 contradicted by the facts of the case, as explained in an elementary manual. It 



* Monthly Microscopical Journal, July and September, 1870. 



+ Quart. Journ. Micr. Sci., July, 1854, P- 212 ; and January, 1855, p. 165, 



