JeflOries.] 94 [July 1, 



Jenks, C. S. Minot and Roger Wolcott, were elected Resi- 

 dent Members. 



Dr. B. Joy Jeffries made the following remarks upon the 

 principle of the Thav;niatrope : — 



Mr. A. Claiidet, F. R. S., about a year ago reported to the Royal 

 Society a new fact in regard to binocular vision, deduced fi-om the 

 thaumatrope. If we print on each side of a card the alternate let- 

 ters of a wcrd, and attaching a string to either end, twirl it around on 

 its axis, the letters on the two sides of the card, ©wing to the con- 

 tinued retinal impression, are apparently seen at the same time. 

 This experiment, carried out in a variety of ways, such as a bhd on 

 one side and a cage on the other, in which it seems to be, forms a 

 popular scientific toy, called the thaumatrope, from two Greek words, 

 meaning "wonder" and "turn." 



Mr. Claudet's observation is this: Let us attach tlie strings by 

 knots at the ends, to prevent their slipping through our holes in the 

 card. Let the two knots be on one side of the card, and now when 

 we twirl it the card turns in the axis of the opposite side of the card. 

 Pulling the strings through, of course reverses the axis to the other 

 side. With binocular vision, the letters on the side away from the 

 observer, i. e. , where the knots are, seem further off, and more indis- 

 tinct. Reversing the knots we reverse the phenomenon. Placing 

 our eye fifteen inches from the card, and supposing the thickness of 

 the card to be J^ of an inch, the difference between the distances of 

 the two surfoces of the card is not more than -^-^^ of the whole dis- 

 tance (fifteen inches). The card I use is about g^^ of an inch, making 

 gig- of the whole distance at fifteen inches. Although there is but 

 this minute difference, yet the phenomenon is evidently due to it, as the 

 experiment readily shows, even to an eye not trained to optical 

 experimentation. This is a new fact in binocular vision, and I admit 

 it as such without hesitation. To my eyes, however, the phenomenon 

 is not simply binocular, but monocular, also, although in less degree, 

 as binocular is better than monocular vision. This, with another 

 point I will bring up, seems to me to destroy ^\x. Claudet's theory of 

 the cause of the above phenomenon. He says, ' ' the efibrt to obtain 

 distinct vision and the effort to obtain single vision act in unison, for 

 it is impossible not to admit that the two muscular processes by which 

 both the angle of convergence is directed to the object, aod the focus 

 of the eyes is adapted to its distance, for the double purpose of hav- 



