15 



II. — An Arrangement for Obtaining Monochromatic Light 



with the Mixed Jet. 



By Edmund J. Spitta, F.K.A.S., &c. 



(Read November 19th, 1902.) 



Since resolution in the Microscope depends upon two functions,, 

 the wave-length of the light used and the N.A. of the objective, 

 it is very obvious, as the latter has reached its practical limit so 

 far as our present knowledge permits us to see, the only hope for 

 increasing the separating power of an objective is to employ mono- 

 chromatic light composed of the shortest wave-length possible. 



The first means which suggests itself to obtain this desirable 

 end is the employment of suitable screens, but it is well known 

 no glass exists, or at present can be made, that is truly mono- 

 chromatic, save perhaps in the red end of the spectrum, which is of 

 no use for the purpose under consideration. Of course, blue-violet 

 glass is made, but what we mean is that, spectroscopically, the 

 screen passes too much light of other colours, usually red. It has 

 been argued, and in a sense truly, that in photography, provided 

 the sensitive plate is not an orthochromatic one, this does not so 

 much matter, seeing the emulsion is not affected by the red ray, so 

 any blurring effect produced by it would not take effect on the 

 plate, and would not be observable in the developed picture. But. 

 the real difficulty lies in the fact that the red image is so disturbing 

 when mixed with the blue-violet, that it seems impossible to focus 

 with the refinement required in photographing minute objects 

 commensurate in size with even perhaps portions of a single ivave- 

 length. If this be true with photography, it is even more so when 

 using the screen for visual work. 



Fluid screens have likewise been tried, but it is found, when 

 they are sufficiently dense to be truly monochromatic, the light i& 

 so feeble that the object — anyhow when the mixed jet is the illumi- 

 nant — is too faint to be well and properly focussed on the focussing 

 screen, especially if the magnification be two or more thousand 

 diameters. 



The employment of prisms of great dispersion has also been 

 well tried — in hands, too, far more able than my own — but they have 

 failed for two reasons. One is, that the light produced is so faint 

 and usually so restricted in area, that the coloured beam is only a 

 narrow strip down the field, even when a -^ homogeneous objec- 

 tive is used ; and the other trouble arises from the fact that neces- 

 sity demands the apparatus to be arranged in a semicircular fashion, 

 and is not capable of being placed in one direct line on an optical 



