24 THE STRUCTURE OF 



the image clearly depicted upon it. Dr. Beale 

 strongly recommends the neutral lens glass reflector 

 in preference to the Wollaston camera lucida. It 

 is also much less costly. (Fig. HA.) This consists 

 of a short tube falling upon the eye- 

 piece, with a piece of neutral tint 

 glass placed at such an angle that, 

 whilst the image of the object is 

 reflected upwards, the paper below 

 can be distinctly seen. (The price of 

 this form of camera lucida is about 

 four or five shillings.) Success in the 

 use of the camera depends very much 

 on the arrangement of the light. If 

 Fig. A. ^ | ma g e j g too stron giy illuminated, 



the paper will hardly be visible; and, on the contrary, 

 if the paper and pencil are too bright, the image is 

 indistinct. A little practice will enable the observer 

 to overcome both difficulties : this he will have 

 attained when he can see the image and paper with 

 equal distinctness. 



Another instrument which will be found of con- 

 siderable service even to the beginner with the Micro- 

 scope, is a micrometer. This is an instrument for 

 measuring the size of objects observed. Exaggerated 

 notions about the smallness of objects are very prev- 

 alent ; and as it is almost impossible to say accu- 

 rately how small an object is without some means of 

 measuring, a Micrometer becomes essential where 

 accuracy is desired. This is effected by having some 

 object of known size to compare with the object 

 observed. The most convenient instrument of this 

 kind is a glass slide, on which lines are drawn the 

 hundredth and thousandth of an inch apart. If this 

 slide, or stage micrometer as it is called, is placed on 

 the stage, the divisions may be traced on the paper 

 in the same way as the outline of an object : the 



