14 Gee and Adamson, Dioptricuieters. 



of such a lens is about 70 mm., and this the instrument 

 will give correctly to the nearest millimetre. With weak 

 lenses, either concave or convex, of about i D, the readings 

 cannot be taken with certainty to less than 1/4 D. With 

 concave lenses of 10 D, the circles are so diminished that 

 the readings can only be estimated to \ D, or 5%. It is 

 better for concave lenses of this kind, or weak convex 

 lenses, to combine with +14 or +15. The auxiliary 

 lens should be placed in the holder below the aperture so 

 as to bring the readings to a better part of the scale. 



The angle of minimum deviation caused by a thin 

 prism or wedge can readily be found by the dioptriemeter. 

 When the prism is placed above or below the aperture, 

 the system of circles is seen displaced to one side. If 

 the prism be rotated about a vertical axis, the circles 

 appear to move round the intersection of the cross wires 

 as a centre. Let the reading on the scale corresponding 

 to this fixed point be P, then the tangent of the angle of 

 deviation will be (20-P)/ioo. 



A prism which causes a minimum deviation of one 

 centimetre when measured at a distance of one metre is 

 said to have a power of one fir is jh dioptrie. By the use of 

 the dioptriemeter the deviation of 20-P mm. measured at 

 a distance of one-tenth of a metre from the prism is 

 clearly the same as a deviation of 20-P cm. measured at 

 one metre, and thus corresponds to a power of 20-P prism 

 dioptrics. 



The instrument may also be used to localise any 

 irregularity in the curvature of a lens at any point. This 

 would be shown, if sufficiently great, by a local distor- 

 tion of the circles. 



When a slab of glass of thickness t is placed at an 

 angle to the axis of the instrument, then the centre of 



