!SS Intelligence and Miscellaneous Articles. 



improbable that any sensible error will arise from this assumption, 

 especially if the thickness of the screen (soot or any other opake 

 medium) may be neglected in comparison with the breadth of the 

 apertures, though a further theoretical examination of the question 

 would be desirable. But both theories differ in what regards the 

 propagation of the secondary waves after their having left the aper- 

 tures. Mr. Stokes assumes that this propagation is the same as if 

 the screen were not there ; indeed he says (in the introduction of the 

 Dynamical Theory of Diffraction), " It is an hypothesis (exceedingly 

 probable d priori) that we may find the disturbance in front of the 

 aperture by merely taking the aggregate of the disturbances due to 

 all the secondary waves, each secondary wave proceeding as if the 

 screen were away, in other words, that the effect of the screen is 

 merely to stop a certain portion of the incident light ; " and further 

 (§ 31 at the end), " In determining the law of disturbance in a se- 

 condary wave we have nothing to do with the aperture." On the 

 contrary, according to the theory of M. Cauchy and that given in 

 my paper, the secondary waves produced by the motion at the issue 

 of the apertures must in proceeding satisfy the condition that the 

 motion resulting from all secondary waves is, on that side of the 

 screen or of the bars of the grating which regards the second me- 

 dium, absolutely zero. It is true that this condition is fulfilled only 

 when the screen is of a substance which absorbs the light in a very 

 high degree (as, for example, soot or silver) ; but for the majority 

 of the substances used as screens the error will not be sensible. 



To this difference in the principles of both theories I should trace 

 the difference of the results ; but after examining that difference, I 

 must give it as my opinion that the assumption of M. Cauchy, while 

 it is confirmed by experiment, is also the true interpretation of the 

 conditions under which the experiment is made ; whereas that of 

 Mr. Stokes, that the secondary waves produced at the apertures may 

 be regarded as proceeding freely in all directions, seems to me to be 

 contrary to these conditions, and the consequences derived from it 

 to be therefore erroneous. 



Yet with the opinion expressed by Mr. Stokes, " that the whole 

 question must be subjected to a thoroughly searching experimental 

 investigation before physical conclusions can safely be drawn from 

 the phenomena," I cannot but concur. 



I am, Gentlemen, 



Yours respectfully, 



Heidelberg, August 14, 1860. F. Eisenlohr. 



ON THE SPECIFIC AND LATENT HEAT OF NAPTHALINE. 

 BY M. ALLUARD. 



Repeated investigations have led Alluard to the following results. 

 Napthaline melts at 79°*9, and solidifies at the same temperature. 

 Its specific heat in the solid form is 03249 between 20° and 66°, 

 and 0*3207 between 0° and 20°; for the liquid condition between 

 80° and 130° it is 04 176. Its latent heat of fusion is 35*68 ther- 

 mal units. Its specific gravity in the liquid state at 99°02 is 0*9628. 

 — Liebig's Annalen, February 1860. 



