320 THE MEASUREMENT OF MAGNIFYING POWERS. 



the objective, only one measurement is required, and that it is 

 quite independent of the power of objective or eye-piece, or of 

 the tube-length ; it can be applied at any moment, without 

 altering any of the adjustments of the microscope, beyond opening 

 out the substage iris to its full extent, and without in any way 

 disturbing the object under examination ; and it is especially 

 easy to apply when dark-ground illumination is in use, as the 

 Ramsden Disc, though faintly illuminated, is then as a rule quite 

 easily seen, and if anything, more easily measured than a bright 

 disc. Its only drawbacks are that it really requires a special 

 instrument of some sort for measuring the R.D. — this, however, 

 is not difficult to improvise — and that when the R.D. is small the 

 accuracy of the method rather falls off, though it is still, unless 

 the R.D. is abnormally small, quite sufficient for all ordinary 

 purposes. For further details, especially in regard to the use of 

 this method when an immersion lens of N.A. greater than unity 

 is in use, I must refer the reader to my letters to the English 

 Mechanic already mentioned. 



I may also mention that this method is analogous to the 

 ordinary method for finding the magnifying power of a telescope, 

 for which M equals A/d, where A and d are the diameters of the 

 object glass and Ramsden disc ; both of these formulae are easily 

 proved by means of what is known as the " sine-law." 



Sown. Qvekctt Microscopical Club, Ser. 2, Vol. XIII., No. 81, November 1917. 



