PHYSIOLOGICAL PHYSICS. [Chap. xxvm. 



would cost him about 9 5s., with the condenser 

 11. 



Some makers designate their objectives by the 

 length of their focal distance. Thus the 1-in. objective 

 magnifies on an average by 80 diameters, J-in. 

 magnifies by 130 diameters, and the ^-in. objective 

 350 diameters. 



To measure magnifying power. The 

 magnifying power is the ratio of the magnitude 

 of the image to the magnitude of the object. Ther 

 are various experimental methods of determining 

 it. For these a MICROMETER is necessary. This is a 

 glass slide on which a series of lines is ruled by means 

 of a diamond, the lines being at stated distances from 

 one another, several being distant y-^th of an inch, 

 several y^Voth of an inch, or it may be y^th and 

 y-Q^th of a millimetre. The micrometer is placed on 

 the stage and focussed. Suppose two lines y^-^th of 

 an inch apart, the question is how far do they seem to 

 be apart when viewed under the microscope. Take a 

 pair of compasses, separate their points and hold them 

 close up to and on a level with the slide on the stage. 

 Both eyes are kept open, the one opposite to the hand 

 holding the compasses looking down the tube of the 

 microscope. With a little practice, one eye will see the 

 image of the lines of the micrometer scale, and the 

 other the points of the compasses. Open or close the 

 limbs of the compass till the images of the two lines 

 coincide with the points of the compass. The distance 

 between the two points is now the apparent distance 

 between the two micrometer lines. The actual dis- 

 tance between the two lines is the yg-^th of an inch. 

 Measure on an inch scale the distance between the 

 two points of the compasses. Let it be \ inch. The 

 apparent distance is ^ inch, the actual distance is 

 yjyo-th of an inch, and the apparent distance divided 

 by the real distance gives the magnifying power. 



