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Prof. J. LeConte on an Optical Illusion. 



I must, however, observe that too much weight ought not to 

 be laid on the approximative values found here for 6 l2 or 9 X and 

 6 2 , since the values a ]2 and a 12 , as I shall take another opportu- 

 nity of arguing more closely, may also, from other grounds 

 than those which have been accepted hitherto, prove to be different. 

 The value given for # 12 with olive-oil and alcohol is too great, as 

 a simple consideration of the drop shows, and a direct determina- 

 tion of 6 V # 2 , and # 12 is always to be preferred to an indirect. 



It may here be mentioned that for liquids which can be mixed 

 in all proportions « J2 =0, — thus for water and aqueous solution 

 of hyposulphite of soda, water and alcohol, oil of turpentine and 

 alcohol, oil of turpentine and olive-oil, oil of turpentine and bisul- 

 phide of carbon. 



If liquid 1 is identical with liquid 2, then a l2 is of course 

 equal to zero. 



The capillary constant a 12 measures the surface-tension of 

 liquid 1 at the common limiting surface just as well as the ten- 

 sion of surface of liquid 2 in the neighbourhood of the common 

 limit. 



[To be continued.] 



XXXIV. On an Optical Illusion. By Joseph LeConte*. 



To Professor Tyndall, RR.S. 



University of California, 

 Dear Sir, Oakland, California, Jan. 27, 1871. 



ONLY very recently my attention has been called to a very 

 interesting and suggestive letter addressed to you by Mr. 

 J. L. Tupperf, containing his explanation of an optical illusion. 

 Permit me through the same medium to make some criticisms 

 on Mr. Tupper's letter, which I hope will place the subject in a 

 clearer light. I am sorry I did not see the letter sooner, as the 

 criticisms would seem more appropriate if they had followed 

 quickly the publication of the original. 



I reproduce here (fig. 1) Mr. Tupper's figure illustrating his 

 experiment, only adding 

 the dotted line to illus- 

 trate my own views. E 

 is a radiant point of 

 intense light, e. g. a 

 pinhole through which 

 light streams, G a larger 

 hole in which is fixed a 

 small pin, and through 

 which a cone of light 

 passes to the eye from E. 

 inverted. 



Fig. 1. 



Under these conditions the pin is seen 

 Communicated by Prof. Tyndall. f Phil. Mag. vol. xxxix. p. 423. 



