Mr. J. L. Tupper on an Optical Illusion. 423 



ticular motion actually taken by the particles to which they are 

 applied; dp and dq are the motions of the particles. Applying 

 this equation to the foregoing case, we get 



i Zmv*= J 2 jj Fdr- J 2 5J- Qdr= §¥dv- JQ^=J (P-Q)<fc. 



It appears, therefore, that, if the identity of the heat evolved 

 by condensation with the vis viva generated by the condensing 

 energy is to be maintained, we must take (P — Q)dv, the effective 

 part of the applied forces, and not Ydv, as the measure of the 

 heat produced. This is in strict conformity with what I have 

 contended for from other considerations. 



Miliaria, May 12, 1870. 



LXI. On an Optical Illusion. By J. L. Tupper*. 



To Professor Tyndall, RR.S. 

 Dear Sir, 



AS the following fallacious " Solution " that appeared in 

 No. 251 of ' The English Mechanic' has not found cor- 

 rection, and as there are points of real interest connected with a 

 true solution of this not new, but, so far as I know, yet unex- 

 plained optical illusion, you will perhaps favour me with your 

 attention to the question. 



u [898.] Optical Illusion. — I beg to forward the following 

 solution : — It is known that if an object be placed at the focus F 

 (tig. 1) of a converging glass, the rays will be so refracted by the 

 glass that, after going out of it, they will become parallel ; if the 

 object be further than the focus (fig. 2), the rays will meet in a 

 point after they have passed the lens; and if nearer, they will di- 

 verge (fig. 3) ; hence results that the rays of light proceeding 



rig. i. 



Fig. 2. 



Fig. 3. 



from certain objects, refracted through the crystalline of the eye, 

 * Communicated by Professor Tyndall. 



