174 SEC. 7. LIGHT. 



796. Lens of Stokes, with constant axis (Snellen) ; con- 

 sisting of a negative and a positive cylindrical lens rotating 

 equally, but in opposite directions. (Graefe's Arcliiv fur Ophthal- 

 inologie, 1873, XIX. 1, p. 70.) Dr. Snellen, Utrecht. 



Two cylindrical lenses (C -^ and C +-5^-) placed centred one before the 

 other can rotate about an axis perpendicular to the plane of the glasses, 

 equally, but in opposite directions, and by means of inclosed steel springs. 

 The principal meridians of the system remain here with the same direction. 

 The refraction in the principal meridians changes with the rotation propor- 

 tionately to the sines of the angle between the axes of the cylinders. Here- 

 upon has been constructed the scale which is to be read on the rim of one of 

 the glasses. For the determination of the refraction, parallel lines are viewed 

 through this lens of changeable power, the lines are perpendicular on one of 

 the principal meridians, and placed either at a distance or united with the lens 

 in a tube. Manufactured by A. Cretes, optician, Paris. 



797. Set of Spherical Lenses, metrical system (by Eoulot). 



Prof. Bonders, Utrecht. 



798. Early Stereoscope, made by the late Sir David 

 Brewster, the inventor of that instrument. John Maclauchlan. 



798a. Early Stereoscopic Pictures, prior to the applica- 

 tion of photography. From the collection of the late Sir C. 

 Wheatstone. Robert Sabine. 



798b. Early Stereoscopic Pictures. Daguerreotypes of 

 1, Biot ; 2, Bequerel ; 3, Foucault, from the collection of the late 

 Sir C. Wheatstone. Robert Sabine. 



798c. Seven Earliest designs for the Stereoscope 



printed in black and white. From the collection of the late Sir 

 C. Wheatstone. Robert Sabine. 



798d. Early Stereoscopic Pictures. Two Daguerreotypes 

 of Faraday, from the collection of the late Sir C. Wheatstone. 



Robert Sabine. 



799. Double Opera Glass. An early example, probably 

 maHe in Holland about 1700. South Kensington Museum. 



801. Galilei's double perspective, No. 17. 



802. The same, No. 29. 



803. The same, No. 37. 



Voigtldnder and Son (Chevalier von Voigtldnder), Brunswick. 



804. Telescope. 



Voigtldnder and Son (Chevalier von Voigtldnder), Brunswick. 



806. A Quartz Ball. A. Hilger. 



