930 SEC. 18. BIOLOGY. 



3710d. Instrument for the determination of the exact 

 position of the chief Meridians of the Eye. 



William Laidlaw Purvcs, M.D. 



This instrument consists of three round apertures, one in the centre, and 

 two at equal distances from the periphery in the same meridian, bored in a 

 screen. When these, placed against a strong light, give major axes of - dif- 

 fusion circles, the disc is revolved till all the major axes are on the same line* 

 This lens has an indicator attached, which points to the degree on a graduated 

 circle in which the disc revolves. (Vide Graefe's Archiv, 1873.) 



3710e. Test Card for determining the Acuteness of 

 Vision in any Meridian. William Laidlaw Purves, M.D. 



This is a modification of the usual astigmatic test cards. By working the 

 disc, and. go bringing varied numbers of lines into any desired meridian, the 

 acuteness of vision at that meridian is determined by the ability of the ob- 

 server to count the number so placed. 



371Oc. Modified Stokes's Lens, for determining the 

 degree and condition of refraction and acuteness of vision of any 

 meridian of the eye, or of simple or combined lenses of less power 

 than one-fifth. William Laidlaw Purves, M.D. 



This is a further modification of Snellen's modification of Stokes's lens. 

 Li this modification the power of the lenses are increased from one-tenth to 

 one-fifth. The constant axis is obtained by means of a screw and wheels, 

 instead of by springs, and, in addition to former modifications, the exact 

 position of the axis of the whole lens is obtained by moving the whole lens in 

 a graduated circle. As each cylindrical lens acts only in one meridian, and 

 that meridian is determined by the position in which the lens is placed, any 

 number of meridians may be determined successively. The power required 

 in any meridian is calculated in the usual way. ( Vide Bonders' Accommo- 

 datipn and Refraction of the Eye.) By using a modification of Scheiner's 

 experiment, and other methods, the power of single or combined lenses may 

 be determined. (Vide Graefe's Archiv, 1873.) 



808. Stokes's Lens, modified by Dr. Snellen. In one and 

 the same mounting are set two piano-cylindrical lenses ( + 12, 12) 

 capable of rotation in opposite directions. When their axes are 

 parallel, their refraction becomes annulled ; when they are at 

 right angles, they become added ; in intermediate positions they 

 give a graduated series of cylindrical refractions. The axis is 

 fixed for the same reasons as above* M. Cretes, Paris. 



3669a. Trial Box and collection of metrical glasses, for 

 ophthalmology. A. Nachet, Paris. 



3717. Set of Lenses for Testing the Sight. 



W. Campbell fy Co.'s successor, J. Wohlers, Hamburg. 



3715. Box_of Lenses, containing 120 spherical, 36 cylindri- 

 cal, 10 prismatic, and two trial eyepieces. M. Roulot, Paris. 



3715a. Box of Lenses, J20 spherical and trial eyepiece. 



M. Roulot, Paris. 



