246 FERGUSON'S LECTURES. 



LECT. ter having crossed each other in the pupil, and passed 

 v^-v"x^ through the chrystalline and vitreous humours they will 

 be collected into points on the retina, and form the large 

 inverted image A B thereon. 



The magnifying power of this microscope is as fol- 

 lows. Suppose the image g h to be six tiroes the dis- 

 tance of the object a b from the object-glass c d; 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 be viewed 

 by an eye-glass ef, of one inch focus, it will thereby 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, makes 36 times ; and 

 so much is the object magnified in diameter more than 

 what it appears to the bare eye ; and consequently 36 

 times 36, or 1296 times in surface. 



But, because the extent or field of view is very small 

 in this microscope, there are generally two eye-glasses 

 placed sometimes close together, and sometimes an inch 

 asunder ; by which means, although the object appears 

 less magnified, yet the visible area is much enlarged by 

 the interposition of a second eye-glass : and consequently 

 a much pleasanter view is obtained. 



The solar The solar microscope, invented by Dr. Lieberkhun, is 

 microscope, constructed in the following manner. Having procured 

 a very dark room, let a round hole be made in the win- 

 dow-shutter, about three inches diameter, though which 

 the sun may cast a cylinder of rays A A into the room. 



