786 SEC. 18. BIOLOGY. 



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. 



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. Webei\ Darmstadt. 



3678. Hose's Schistoscope, for the physiology of colour. 



F. Schmidt and Haensch, Berlin. 



3679. Micrometer, for measuring objects in the fundus oculi. 



Dr. Laqueur, Strasburg. 



This apparatus is used for measuring in the living eye the real size of the 

 " papille " blind spot, the diameter of the blood vessels of the retina, the 

 size of the macula lutea, the distance of the latter from the external edge 

 of the blind spot, also the dimensions of any extravasations of the retina. 



For the method of using the apparatus, see the accompanying paper: 

 Centralblatt der med. Wiss. 1 873, No. 59. 



It can also be used for determining the condition of the refrangibility of 

 the eye in ophthalmoscopic examination, by placing the toothed part of the 

 stems in front of the lamp which serves as a source of light. 



An account of the apparatus is published in the Proceedings of the Oph- 

 thalmoscopic Society at Heidelberg for 1875. 



3680. Dr. Forster's " Lichtsinnmesser." 



R. Sitte, Breslau. 



A source of light of constant intensity, but whose size can be varied, 

 illuminates the object, which may be either black bands or large letters on a 

 white ground. The source of light is surrounded with white paper, which is 

 illuminated on the other side by a standard candle burning at almost the 

 same height. The size of the source of light depends on an adjustable 

 diaphragm, whose centre always remains at the same point. The diagonal of 

 the square aperture of the diaphragm is measured by a scale which moves 

 with the plates of the diaphragm. The size of the illuminated surface 



d 2 

 ~ , if half a square millimeter be taken as the unit. Thus one can learn 



by means of this apparatus all quantities of light, from 1 to 2,500 units. 

 See Zehender's Klinische Monatsblatter fur Augenheilkunde, 1871, p. 337. 



368Oa. Dr. Forster's Perimeter. R. Sitte, Breslau. 



This apparatus is used for determining the external limit and the colour 

 limit of the defects of the field of vision. The position of the displaceable 

 objects is read off by meridians and parallels. 



