78 Modern Microscopy 



for them; but it should be borne in mind that English 

 manufacturers give in many instances as deep a power of 

 eyepiece as 20 or 25, whereas Continental manufacturers 

 rarely supply them of greater magnifying power than 10 or 

 12 diameters. In the English series the variation in power 

 between two consecutive eyepieces is generally greater than 

 in the Continental series. A comparison, therefore, between 

 the merits of an English object-glass tested with, say, a 

 * D ' or No. 4 English eyepiece and a Continental object- 

 glass of the same power tested with a No. 4 eyepiece of 

 Continental make would not be fair, as the former, having 

 a deeper power eyepiece on it, would be liable not to give 

 such perfect results as the latter. And here we may men- 

 tion that, although people very often buy deep-power eye- 

 pieces, it is advisable, with ordinary achromatic lenses, that 

 no stronger power should be used than an eyepiece giving 

 an initial power of 10 or 12 diameters. The best eyepiece 

 for general purposes is the * B.' This gives a convenient 

 size of field, and is by far the most comfortable to work 

 with of the whole series. Next, and in addition to this, we 

 should recommend either the ' C ' or the ' D.' 



Microscopists having abnormal vision, and preferring to 

 work without spectacles, should have an auxiliary cap made 

 to fit over their eyepieces, carrying a lens of the power 

 that corrects the error of vision. This is especially neces- 

 sary where measuring has to be done or where the micro- 

 scope is arranged for a second person's inspection. 



At times it is desired to know whether an eyepiece can 

 have its diaphragm enlarged so as to give a larger field. 

 An easy method of ascertaining how much of the field lens 

 is employed is to make a spot with ink near the margin 

 on the convex side of the field lens, and on placing the eye- 

 piece in the microscope, if the diaphragm has a sufficiently 

 large aperture, the ink will be visible ; if not, it may be 

 enlarged until it appears. The diaphragm should not be 

 eo large as to admit of more than the edge of the field lens 

 being visible. 



