II. OPHTHALMOLOGICAL. 93 I 



371 5b. Box of Lenses, 36 cylindrical and eyepiece. 



M. Roulot, Paris. 



3715c. Box of Lenses, from 2 to 20, square, cylindrical. 



M. Roulot, Paris. 



c. PERIMETERS. 

 ' 3714b. Dr. Badal's Perimeter. M. Roulot, Paris. 



3680a. 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. 



A more detailed description will be found in Zchender's Klinischen Monat- 

 schriften fur Augenheilkunde, 1869, p. 411. 



3711. R. Brudenell Carter's Perimeter for ascertaining 

 the boundary of the field of vision and the area of the blind 

 spot. T. Hawksley. 



d. PHOTOMETERS. 



3695. Graefe's " Leuchtscheibe " (illuminating disc) ; 

 from the property of the late Professor von Graefe. 



Dr. Weber, Darmstadt. 



3680. Dr. Porster'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 behind a sheet of white paper, and 

 shines only through it, the paper being 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 ihe 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 



rf 2 

 size of the illuminated surface =^, if half a square millimeter be taken 



as the unit. By means of this apparatus all quantities of light, from 1 to 

 2,500 units, can be estimated. 



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



3720. Weber's Photometer and Chromometer. 



Dr. Weber, Darmstadt. 



3721. Weber's Chromoptometrical Apparatus. 



Di\ Weber, Darmstadt. 



The examination of the sense of colour with this apparatus rests upon the 

 production of simultaneous contrasts, for which purpose a gray ring is placed 

 upon the primary colour and both are covered with transparent paper. For the 

 determination of the numerical size of the perception of colour, use is made 

 of a gray disc which can be divided into sections, by means of which it can 

 be determined in what relation the primary colour stands to the gray surface, 

 in order to place upon it likewise the complementary colour. For determining 



3N 2 



