18 CONSTRUCTION OF 



actual manufacturer of the microscope alone would this information be 

 of interest ; and as this little Manual is intended rather for the use of 

 the amateur and general reader, and has been written with the view of 

 conveying a knowledge of the manipulation of the instrument, and the 

 method of preparing and classifying objects for it, we shall content 

 ourselves with a brief enumeration only of the optical principles on 

 which it is constructed, dwelling more largely upon its use, and its 

 wondrous revelations/'* 



CHAPTER II. 



CONSTRUCTION OF MICROSCOPES. 



7. The forms of microscopes are very numerous, but they may all 

 be included in two distinct classes, however much they may vary as to 

 their external appearances, viz. : SINGLE MICROSCOPES, in which the 

 object itself is magnified, whether by a single lens, or a combination of 

 lenses; and COMPOUND MICROSCOPES, in which a magnified image of 

 the object, not the object itself, is magnified. 



8. To comprehend how it is that the lenses, which are used in the 

 formation of all descriptions of microscopes, increase the size of 

 objects, or magnify them, as it is termed, the reader must clearly under- 

 stand what is meant by the apparent magnitude of objects. If a small 

 coin be placed at the distance of a hundred yards from us, it will be 

 scarcely perceptible ; and its apparent magnitude, or the angle under 

 which it is seen, is said to be then extremely small. At the distance of 

 twenty or thirty yards we should just perceive that it was a round 

 body, and that its apparent magnitude had increased ; at the distance 

 of three yards, we should begin to trace the effigy and inscription upon 



* To those who may be desirous of studying this question more in detail, we 

 would recommend for consultation, Brewster's Treatise on Optics; Article Mi- 

 croscope, in the Penny Cyclopcedia, and the same Article in the ' Edinburgh Gyclo- 

 paedeia,' and the ' Encyclopaedeia Britannica.' 



