24 THE STRUCTURE OF 



of the prism, the other sees the paper below with 

 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 ot 

 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 

 Fig. 11 A. much on the arrangement of the light. 

 If the image is too strongly 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 

 Microscope, is a micrometer. This is an instrument 

 for measuring the size of objects observed. Exag- 

 gerated notions about the smallness of objects are 

 very prevalent ; and as it is almost impossible to 

 say accurately how small an object is without some 

 means of measuring, a Micrometer becomes essen- 

 tial where accuracy is desired. This is effected by 

 having some object of known size to compare with 

 the object observed. The most convenient instru- 

 ment 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 



