156 THE AMERICAN MONTHLY [Apr 



in recent years to give large apertures at a moderate 

 cost, makers have been so unscrupulous as to state the ap- 

 ertures of their objectives to be far in excess of that 

 which they actually possess. This is fraud pure and sim- 

 ple, and it is a temptation which all opticians should have 

 principle enough to withstand. Associated with this 

 misrepresentation question is another of only slightly 

 less importance ; one is, in fact, the outcome of the other. 

 Many objectives are stated to be of a certain focal power, 

 but on testing, it will very frequently be found that they 

 magnify considerably more than they should do. Thus a 

 1-6 in., which should yield a magnification of 50 diam- 

 , eters at an image distance of ten inches, will probably 

 prove to magnify about 70 diameters. It is obviously 

 easier to give a larger aperture with an increased mag- 

 nification in the objective, but it is no satisfaction to a 

 worker to purchase an ostensible 1-6 in. lens of, say, .85 

 N.A., to find that it is actually a 1-7 in. of .80 N.A. or 

 thereabouts. It is imperatively necessary that the pow- 

 ers and apertures of objectives be stated as nearly as 

 possible. 



Uniformity and accuracy are more than desirable in 

 connection with the microscope, and the opinions express- 

 ed in this short note are those of a large section of work- 

 ing microscopists in all parts of the world. — i". A. M. 



Magnesium Light in Photo-Micrography. 



PROCEIXA. 



I have practiced photo-micrography for the last twenty- 

 five years. I began a considerable time before the intro- 

 duction of the modern dry plate in 1879-'80. Collodion 

 was the sensitive medium then used, and as my business 

 duties only permitted the necessary leisure in the even- 

 ings the practice was pursued under considerable difficul- 

 ty — the chief one being the illuminant. 



