70 Modern Microscopy 



powers would be much more severely tested than if employed 

 at 6 inches. 



It would be a great advantage to the microscopist if 

 opticians would mark exactly the focal power and precise 

 numerical aperture of their objectives upon them. In 

 order that objectives may appear to have a large ratio of 

 aperture to power, they are often put forward as possessing 

 a considerably lower power than they actually have. For 

 instance, a so-called 1-inch often turns out to be nearer 

 |-inch, J-inch about rV-i^icl^) ^-inch to have the power 

 of ^-inch, and xV-i^ich in some instances to be xV"i^ch. 

 It has become such an acknowledged fact that the act 

 of misrepresentation involved seems to be condoned. 

 This is a state of things which should not be. Opticians 

 must be aware of the misdescription and the immensity of 

 trouble that is caused by it. We must, therefore, advise 

 microscopists not to rely on the powers marked on their 

 objectives, but to ascertain them for themselves, and the 

 best way to do it is to project the image of a micrometer, 

 without any eyepiece in the body-tube, on a screen 20 inches 

 distant from the back lens of the objective. Measure with 

 a foot rule the distance apart of the lines so projected, 

 and supposing that each hundredth of an inch measured 

 on the screen 1 inch, that would represent a magnification 

 of 100 diameters ; divide the distance used (20 inches) by 

 the magnification found (100 diameters), and the result 

 (tttV or -^-inch) is the equivalent focus or 'power' of the 

 objective. 



TESTING OBJECTIVES. 



It is a somewhat difficult matter for the novice to decide 

 for himself as to the quality of object-glasses. Such work 

 needs experience, judgment, and a trained eye. The writer 

 has met with people who have not been able to distinguish 

 the difference in performance between an uncorrected single 

 French lens and a first-class achromatic. This, of course. 



