8 Gee and Adamson, Dioptrienutcrs. 



The instrument — which may be called a Dioptrie- 

 meter — is shown in Fig. 5. It consists of a wooden 

 upright supported on a wooden base. On the latter is 

 fixed a horizontal card, marked with a series of concentric 

 circles i mm. apart, the radius of the innermost circle being 

 5 mm., and that of the outermost 30 mm. A thin disc of 

 metal, with a circular aperture of 10 mm. radius, is supported 

 exactly lOO mm. above the card. Another thin disc of 

 metal, having a central pin-hole, is fixed exactly 200 mm. 

 above the card. The centre of the circular opening is 



Fig. 5.— The Dioptriemeter. 



indicated by cross wires, and the centre of the concentric 

 circles is marked by a cross. These two centres and the 

 pin-hole lie in the same vertical line, which is the axis of 

 the instrument. 



When the eye is placed close to the pin-hole the field 

 of view limited by the circular aperture will obviously 

 include all the circles up to that of 20 mm. radius. This 

 circle is called the zero circle, and is marked o. The 

 other circles are numbered consecutively, those inwards 

 being marked 4-, those outwards — . 



