932 SEC. 18. BIOLOGY. 



the limit of the field of perception of colour, use is made of a number of con- 

 centric gray rings, for which the middle point of the prime-coloured surface 

 serves as a centre which is fixed upon by the eye under examination. 



. TONOMETEES. 



3675. Graefe's Tonometer (Augendruckmesser). 



Dr. Weber, Darmstadt. 



Graefe's tonometer has only an historical value, since it was the first appa- 

 ratus with which he endeavoured to measure the tension of the eye-ball 

 tegument ; it could not, however, be introduced into practice on account of 

 many inconveniences. 



3676. Weber's Tonometer. Dr. Weber, Darmstadt. 



3674. Tonometer, for measuring the Tension and 

 Convexity of the Eye-ball. 



Geneva Association for the Construction of Scientific In~ 

 struments. 



This apparatus is specially used for glaucoma. It serves to produce a definite 

 deformation of the eye, which may be variable at the operator's will, according 

 to the most suitable circumstances for a special case. 



The instrument is composed of a rod, of which the ivory-tipped end presses 

 upon and deforms the eye, while the other end acts upon a dynamometric 

 spring, the flexure of which, amplified by a catch and by the indicating needle, 

 shows in grammes upon the dial the pressure exercised upon the rod, and, 

 consequently, upon the eye. 



An index marks the greatest oscillations of the principal needle. 



/. PUPILLOMETEKS. 



3671. Pupillometer. Emil Stohrer, Leipzig. 



g. EXOPHTHALMOMETERS. 



3682. Cohn's Exophthalmometer, three specimens. 



Prof. Dr. H. Cohn, Breslau. 



These instruments possess a certain amount of interest, as they are almost 

 the first instruments which were made for measuring the prominence of the 

 eye from the orbit. Dr. Cohn exhibited the first of the accompanying instru- 

 ments, on Nov. 17th, 1865, before the Medical Society of Breslau, and at the 

 same time gave it the name of Ophthalmoprostatometer ; but later, in the 

 year 1867, he changed the name to Exophthalmometer. The second in- 

 strument Dr. Cohn has not publicly demonstrated, as he altered the principle 

 of measurement; nevertheless he determined to send the instrument, so 

 that one can judge of the reasons which induced him to regard the external 

 'wall of the orbit which he had taken as his fixed point (and is still so taken 

 by some) as not suitable for the purpose. The third exophthalmometer 

 with which the exhibitor finally carried out his measurements, was laid 

 before the Ophthalmic Congress, at Paris, on the 12th of August 1867. Its 

 peculiarities and applications are described in the accompanying pamphlet. 



3 6 82 a. Cohn's Exophthalmometer, with band. 



Prof. H. Cohn, Breslau. 



