680 SUMMAEY OF CURRENT RESEARCHES RELATING TO 



A Fig. 123. ff 



Fig. 124. 



horizontally on the table, this 

 camera lucida certainly shows 

 the drawing-pencil with greater 

 distinctness than any other 

 similar instrument, that of 

 Oberhauser-Hartnack not ex- 

 cepted. For rery long-sighted 

 persons the two convex glasses 

 placed below the two smaller 

 mirrors may be of use, but for 

 normal and short-sighted per- 

 sons they are useless, and it is, 

 moreover, better left to each 

 individual to assist his sight 

 by spectacles as required. The 

 cap (with an aperture) over 

 the two mirrors is also well 

 adapted to serve as a guide to 

 direct the eye of the observer, 

 and thus facilitates its use by 

 beginners, who often have a 

 difficulty in finding the image. 

 These advantages are, how- 



receives the rays from S and reflects 

 them upon a plate of glass s 1 and 

 thence to the eye, the pencil being 

 seen through the latter (abed is 

 the fitting which holds s 1 and s 2 ). 

 There is a subsidiary apparatus 

 formed of two plano-convex lenses 

 for reducing the amplification. 



Oberh'auser's (or Hartnack's) 

 camera is shown in Figs. 123 and 

 124. It consists of two tubes A A 

 at right angles, a rectangular prism 

 d being inserted at the point of 

 junction, by which the rays coming 

 from the object are reflected through 

 an eye-piece B fe to a smaller prism 

 0, and thence upwards to the eye. 



