368 PHYSIOLOGICAL PHYSICS. [Chap, xxvin. 



use, and for the position of the draw-tiibe, at the time 

 when the determination is made. 



To measure the actual size of the object. 



If one had determined the magnifying power of the 

 microscope and then focussed the object, the actual 

 size would be known by measuring with compasses or 

 scale the apparent size. The apparent size divided by 

 the magnifying power gives the actual size. Thus, if 

 the apparent size were 1 inch and the magnifying 

 power 300 diameters, the real size would be -anoth 

 inch. This method, however, is not quite exact. A 

 more correct method requires a micrometer scale for 

 both stage and eye-piece. The stage micrometer has 

 been already described. The eye-piece micrometer 

 consists of a piece of glass having fine lines, equi- 

 distant from one another, drawn upon it, and it is of 

 great advantage that every fifth line should be longer 

 than the other four. This micrometer may be in 

 the form of a circular piece of glass fitted into a 

 piece of tube of proper length, arranged for dropping 

 into the eye-piece by unscrewing the eye lens, which 

 is then replaced. The micrometer tube rests on the 

 diaphragm of the eye-piece, and ought to be of a 

 length to permit of the lines being in proper focus. 

 Or the eye-piece micrometer may be on a slide which 

 is slipped into the eye-piece by a slit in the side. 

 It being adjusted, the lines on the stage micrometer 

 are brought into focus, the lines of the eye-piece 

 micrometer then appear superimposed on those of the 

 stage micrometer, and it is found how many divisions 

 of the former are equal to one of the latter, which is 

 equal, let us say, to the T^oo^ n f an inch. Suppose 

 five of the eye-piece divisions were equal to one of the 

 stage divisions, then each line of the eye-piece micro- 

 meter is distant from the other the -g^Qth of an inch. 

 Now remove the stage micrometer, and place on the 

 stage the object to be measured. On focussing it will be 



