OPTICS. 271 



The appearance of the image in the air, between 

 the mirror and the object, has been productive of 

 many agreeable deceptions, which, when exhibited 

 with art, and an air of mystery, has been very suc- 

 cessful, and a source of gain to many of our public 

 show-men. 



OF MICROSCOPES. 



The word microscope signifies an instrument for 

 viewing small objects. 



It is usual to say, that the microscope magnifies 

 objects seen through it : the reason of this will be 

 seen by referring to what has been said respecting 

 the optic angle. 



The apparent magnitude of objects is measured 

 by the angle which they are seen under by the 

 eye ; and those angles are reciprocally as the dis- 

 stances from the eye. If, therefore, at the dis- 

 tance of six inches, we can but just discern an 

 object, and then by interposing a lens, or other 

 body, we can view that very object at a nearer 

 distance, the object will appear to be as much 

 larger through the lens than before to the naked 

 eye, as its distance from the lens is less than its 

 distance from the eye. 



That this is the case, is evident from Plate 13. 

 fig. 8, where A is a point in an object not clearly 

 visible to the naked eye at a less distance than A B, 

 because the rays which proceed from it are too 

 divergent to admit of distinct vision till they have 

 passed that distance ; but if the same object be 

 placed in the focus C of the lens D (Fig. 9), the 

 rays which proceed from it will become parallel, 

 by passing through the lens, and, therefore, the 



