Experimental Philosophy. [Lecture 13. 



The magnifying power of this microscope is 

 as follows. Suppose the image gh to be six times 

 the distance of the object db from the object-glass 

 cd ; then will the image be six times the length 

 of the object : but since the image could not be 

 seen distinctly by the bare eye at a less distance 

 than six inches, if it is viewed by an eye-glass ef, 

 of one inch focus, it will be brought six times 

 nearer the eye ; and consequently viewed under 

 an angle six times as large as before ; so that it 

 will be again magnified six times ; that is, six 

 times by the object-glass, and six times by the 

 eye-glass, which multiplied into one another 

 make thirty-six times ; and so much is the ob- 

 ject magnified in diameter more than it appears 

 to the bare eye; and consequently thirty-six 

 times thirty-six, or one thousand two hundred 

 and ninety-six times in surface. 



The solar microscope is constructed upon si- 

 milar principles. Two convex glasses are in- 

 closed at their proper distances in a brass tube. 

 This tube being fixed in the window-shutter of 

 a dark room, the object is put between the two 

 glasses, when a very large inverted image of 

 it will be exhibited on the opposite wall, pro- 

 vided the sun shines sufficiently bright and clear 

 upon the microscope. This instrument bears a 

 strong analogy, therefore, to the camera obscura 

 already described. Sometimes, three lenses are 

 employed, and the magnifying power of the mi- 

 croscope proportionally increased. 



