II. OPHTHALMOLOGICAL. 933 



3682b. Cohn's Exophthalmometer, with small handles. 



Prof. H. Cohn, Ereslau. 



h. OPHTHALMOMETERS. 



371Of. Ophthalmoscope, for the determination of the re- 

 fraction of the observed eye by the observer. 



William Laidlaw Purves, M.D. 



Behind the mirror are two discs containing numerous convex and concave 

 lenses, which may be brought rapidly over the aperture of the mirror by 

 means of a rack moved by the thumb of the hand which holds the ophthal- 

 moscope. As the convex can be revolved over the concave lenses, or vice 

 versa, any combinations can be made which the number of the lenses em- 

 ployed will allow of. 



3710g. Ophthalmoscope, for the determination of the re- 

 fraction of the observed eye by the observer. 



William Laidlaw Purves, 'M.D. 



Behind the mirror two cylindrical lenses, + and , are revolved on 

 the same method as in Stokes's lens, and thus the powers, which have been 

 calculated and marked on the instrument, are brought into use instead of 

 using a number of lenses as in the instrument before described. 



3681. Cohn's Refracting Ophthalmoscope. 



Prof. Dr. H. Cohn, Breslau. 



In the year 1871 the exhibitor constructed this apparatus for examining as 

 quickly as possible a number of scholar's eyes. In 1872 the instrument was 

 described in the Klinischen Monatsbl. of Augenheilkunde, in the October 

 number. Twenty-four glasses can be quickly pushed past behind one and 

 the same mirror. As the disc which bears these glasses is centred above 

 the mirror, the nose of the observer cannot come into contact with it. 

 There is no necessity to shift three discs with glasses, as in Loring's Ophthal- 

 moscope. If the mirror be placed in front of the eye to be examined, and the 

 patient looks through the central hole at some letters in the distance placed 

 oa an inclined table desk (as Snellen's alphabet), the instrument can be used 

 instead of a test lens, and thus the wearisome selection of the glasses is 

 avoided ; in the same way, by quickly turning the disc with the correction 

 glasses, its refrangibility for an object can be removed. 



3708. C. J. Oldham's First Ophthalmoscope, with one 

 diaphragm at back of mirror to carry lenses. T. Hawksley. 



3709. C. J. Oldham's Second Ophthalmoscope, with 

 three diaphragms and 27 lenses. T. ffawksley. 



3719. Metrical Ophthalmoscope, of Dr. de Wecker. 

 In one zone are placed 20 convex glasses ascending gradually 

 from 0*50 to 10 metrical dioptric. To obtain the concaves, a 

 concave lens is interposed, more powerful than the maximum of 

 the convex glasses (10J dioptrics), and by turning the wheel a 

 descending series of 0' 50 to 10*50 concave dioptrics is obtained. 



M. Cretes, Paris. 



