180 TESTING THE MICROSCOPE. 



then be applied directly upon a finely divided scale. The limiting 

 lines can also be drawn repeatedly upon the plane of projection 

 with the fine point of a pencil, and the mean value reckoned as 

 the diameter of the virtual image. If this diameter amounts, for 

 instance, to 26*5 mm., and that of the object to 100 mic. = *l mm., 

 the magnifying power m =265. 



Where an accurate determination of the magnification is required, 

 all the conditions which theory presents must be satisfied as far as 

 possible. The magnitude of the object should then be determined 

 in a more reliable manner than is possible by means of the micro- 

 metric divisions (which are not always exact). For this purpose, 

 Harting proposes to wind a thin metal wire some hundred times 

 round a thicker wire so that each revolution exactly touches the 

 preceding one which must be verified by the Microscope. We then 

 measure the space which the whole of the revolutions occupy on 

 the thicker wire, and finally count the number of revolutions, by 

 unwinding it on a lathe. The total length of the windings, divided 

 by their number, gives the thickness of the wire with an accuracy 

 which is not attainable by micrometric measurements. Such a 

 wire can either be viewed as the object, or used for testing the 

 exactness of the micrometric divisions. 



In practice, however, such extreme care is generally superfluous, 

 It is unimportant whether the magnification differs by a few units, 

 more or less whether it is, for instance, taken as 360 or 355 for 

 the objects to be measured and represented always differ in magni- 

 tude by more than this amount. We need not, therefore, pay 

 much regard to the second focal point of the Microscope, the 

 position of the eye, the accuracy of the divisions of the micro- 

 meter, &c. ; in general it will suffice if we measure the distance of 

 distinct vision starting from the middle of the eye-piece, and 

 bestow the usual amount of care upon the determination of the 

 relative magnitudes. 



The magnifications, which we obtain by the different objectives 

 with the same eye-piece, are, of course, proportional to the linear 

 dimensions of the real objective-images. If, for instance, the 

 image of one objective covers ten divisions of a micrometer in 

 the eye-piece, and that of another objective fifteen divisions, the 

 magnifications are in the ratio of 10 : 15. If, conversely, a micro- 

 meter-division is viewed as the object, the number of the divisions 

 which are seen within a given space (for instance, the field of 



