10 THE STRUCTURE OF 



your left right. The observer, however, soon gets 

 accustomed to this, and it creates no difficulty ulti- 

 mately. But science constantly attends on the 

 Microscope, and ministers to its slightest defects. 

 A little instrument called an erector, composed of 

 a lens which reverses the picture once more, is 

 supplied by the optician, and can be had by 

 those who practise the refinements of microscopic 

 observation. 



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, which is called a stage 

 micrometer, is laid over an object, or the object 

 placed upon it, its relation to the ruled lines will 

 be easily seen, and the size computed accordingly. 

 Many other forms of micrometer have been in- 

 vented, but this is one of the simplest and most 

 easily used. 



It is a good plan to make drawings of all objects 

 examined, or at any rate those which are new to 

 the observer. A note-book should be kept for 

 this purpose, and what cannot at once be identified 

 by the object, may afterwards be so by the drawing. 

 All persons, however, have not the gift of drawing, 

 and for those who need assistance in this way, the 

 camera lucida has been invented. This instru- 



