484 JAMES CLERK MAXWELL. 



Light," the subject which had interested him so much in 

 1847 (see Part I., p. 84), in the Transactions of the Eoyal 

 Society of Edinburgh for 1872; and on " Double Eefraction 

 in a Viscous Fluid in Motion," in the Proceedings of the 

 Royal Society for 1873, and the Ann. de Phys. and Chimie, 

 1874. 



Among the many optical contrivances designed by 

 Professor Maxwell, we ought not to omit to mention the 

 real-image stereoscope. This instrument, in which ordinary 

 stereoscopic slides are employed, consists essentially of two 

 convex lenses of short focal length, say 4 inches, placed side 

 by side in a wooden frame at a distance from the pictures, 

 equal to twice their focal length, while the distance between 

 the centres of the lenses is half the distance between the 

 centres of the pictures. The result of .this arrangement is 

 that real images of the two pictures of the same size as the 

 pictures themselves, formed one by one lens, and the other 

 by the other, are superposed on the axis of the instru- 

 ment at a distance in front of the lenses equal to twice their 

 focal length, that is, as far in front of the lenses as the pic- 

 tures are behind them. The double, or stereographic image 

 so 'formed is viewed through a large lens at a suitable dis- 

 tance in front of it, the observer standing at a distance of 

 three or four feet. Many persons * who cannot appreciate 

 the ordinary box stereoscopes obtain very satisfactory effects 

 with this instrument. 



Professor Maxwell prepared a large number of stereo- 

 scope slides of geometrical figures. Among them may be 

 mentioned the surface of centres of an ellipsoid, lines of 

 curvature on an ellipsoid, and on elliptic and hyperbolic 

 paraboloids, the parabolic cyclide, the horned cyclide, and 

 the spindle cyclide, a twisted cubic with three asymptotes, a 

 Gordian knot, etc. The wood blocks from which these 

 slides were printed are now in the Cavendish Laboratory. 



Another extremely pretty optical toy of his construction, 

 at present in the possession of Mrs. Maxwell, is a Zoetrope, 



1 Including the present Lord Bishop of Durham. 



