FUTUKE OF THE MICROSCOPE 41 



work under observation. Further, it is possible, by interposing 

 neutral screens, to vary the light intensity if the electrical method 

 is inconvenient. Owing to its possessing practically no red radia- 

 tions, its mean wave-length is shorter, and by using suitable screens 

 light which is truly monochromatic, green, blue or violet, can be 

 obtained at will. These lamps are made both in glass and quartz, 

 but the quartz ones are preferable, because they admit of the use 

 of heavier current, with greater luminosity; and further, they have 

 a much longer life. I have exhibited two of these lamps, because 

 1 regard them as far in advance of any other form of light avail- 

 able tc the microscopist at the present time, whether he is a biolo- 

 gist or a metallographer. 



The whole subject of illumination, so far as the illuminant is 

 concerned, needs investigation also, because there is, I think, little 

 doubt that a modification in the intensity of the illumination of 

 any particular object enables us to use a larger light cone than 

 we could do under ordinary circumstances. That is, variation of 

 the intensity is an alternative to the use of the iris diaphragm in 

 the sub-stage of the microscope. But it is in the direction of using 

 invisible radiations in the ultra-violet, or, it may be, radiations 

 which are still shorter than the ultra-violet, that developments in 

 microscopic work are, in my opinion, likely to occur. 



There are two other points, which I can only refer to, but which, 

 I trust, may be dealt with more fully in the succeeding papers. 

 One is that, while the resolution limits are so inflexible, that does- 

 not by any means apply to mere visibility. By illuminating small 

 particles by means of an annular cone of rays, that is, what is 

 ordinarily known as dark ground illumination, or by illuminating 

 them at right angles to the optic axis of the microscope — what is 

 known as the ultra-microscopic method — particles of a very much 

 smaller order of siz.e can be made visible. But we cannot tell any- 

 thing about their form, nor can we accurately tell their size. We 

 are only conscious of their mere existence. 



Another point to remember is that magnification is definitely 

 limited to something like 750 diameters with microscopes under 

 ordinary conditions, if we want to get the best optical effect. We 

 may, as a matter of convenience, have still higher magnifications, 

 because it is not given to everybody to appreciate fine detail unless 

 an image is somew^hat enlarged. But it must be appreciated that 

 any increase beyond 750 or 800 diameters does not result in us 

 seeing anything more. It simply allows us to see the object on a 

 somewhat larger scale. We may therefore summarise as follows: — 

 An object which is much smaller in size than the resolution limit 

 can be rendered visible, providing the light with which it is illuiu- 

 inated is of sufficient intensity, and it is sufficiently different in 

 refractivity from the medium in which it lies. To resolve a series 

 of equi-distant points or lines in an object, their distance apart 

 must exceed half a wave-length of light in the medium in which 

 the object is immersed. Johnstone Stoney has shown that a pair of 

 lines or objects can be separated when their distance apart is rather 

 smaller than the resolution limit required for a number of points 

 or lines in a row. But it should be borne in mind even here that 

 the resolution limits apply if a definite standard of definition is 

 required. An isolated object, or pair of objects, are not so well 



