SOME ACCESSORIES AND THEIR USES 43 



where it can be drawn, preferably with a hard 

 pencil. The novice wiU probably experience a httle 

 difficulty in hie first use of the camera lucida, but 

 he will gain the knack of it with practice. It is 

 necessary to keep one-half of the pupil of the eye 

 looking upon the edge of the camera, and the other 

 half at the pencil and paper. Light also must be 

 carefuUy adjusted, as much depends upon the 

 brightness of the image appearing on the paper in 

 relation to that of the paper and pencil. 



To ascertain the degree of magnification of an 

 object by the microscope in a sufficiently exact 

 manner, a Stage Micrometer is needed. This acces- 

 sory is a glass shde ruled with a fine, divided into 

 spaces either y^ or y^ij inch wide, or possibly 

 marked in fractions of a millimetre. It is used thus : 

 The microscope is arranged for drawing with the 

 camera lucida attached as usual, care being taken 

 that the distance of 10 inches between tube and 

 paper is maintained. The object is focussed, and 

 the dimensions of the image marked on the paper. 

 Then the stage micrometer is substituted for the 

 object, and the image of its divisions seen on the 

 paper gives an indication of magnification, as well 

 as of the actual size of the object. Such a ruled 

 glass micrometer costs 5s. 



A Live Cage is used for holding small living objects 

 in position, and if properly handled wiU hold many 

 live creatures in position without destroying them. 

 This accessory is particularly useful in examining 

 pond life, such as water-fleas, cyclops, and water- 

 mites. It consists of a brass plate, with a circular 

 aperture crowned by a short brass tube, holding a 



