312 The Outline of Science 



parts, e.g. in the quality of the glass used in making the lenses; 

 and a present-day microscope is certainly a very perfect instru- 

 ment. Indeed, unless some new idea is discovered, such as those 

 behind the ultra-microscope and dark -ground illumination, it does 

 not seem likely that great advances in technical microscopy can be 

 made. The reason for this statement is to be found in the optical 

 limitations of the instrument. The use of the microscope is not 

 mainly magnification but resolution. "By resolution," says Mr. 

 J. E. Barnard, "is meant the power the objective has of separat- 

 ing and forming correct images of fine detail." Unless we see 

 more of the intimate structure, the magnification in itself does not 

 greatly avail. It does not help us to understand the thing better. 

 Now there are two factors that determine this "resolving" power 

 of the microscope. The first is what is called the "numerical aper- 

 ture" of the lens, which means, in a general way, the number of 

 divergent rays of light that the curvature of the lens will allow 

 to impinge upon it. Lenses of high magnifying power are so 

 small that they admit only a very small beam of light. Thus what 

 is gained in magnifying power may be lost because of deficient 

 illumination. A pretty device to increase the income of light in 

 these high-power lenses was the "immersion lens," made of such 

 a curvature that when the lens was focussed down into a drop of 

 oil, or some other liquid, placed over the object on the slide, it 

 received light from all sides. The drop into which the lens is 

 focussed down or "immersed" greatly increases the illumination 

 of a lens with high magnifying power. This method has enhanced 

 the value of the microscope as an instrument that analyses struc- 

 ture, or, in other words, that discloses the intimate architecture of 

 things. But the main point is that the "numerical aperture" of 

 even the oil-immersion objective has at the present time reached 

 its practical limit. 



Yet there is a second factor, and that is the wave-length of the 

 light-rays that impinge from the mirror and condenser on the 

 object on the slide. But here again there is a limit, for, as Pro- 



