Note. 201. 



the spectra a, p, y are inaccessible, even with oblique light, or at least 

 so much weakened in their intensity, that no effect will get visible. 

 An aperture of 1 ' 25, on the contrary, admits, with oblique light, one 

 of the spectre of the second circle with full intensity : in looking down 

 into the open tube, th.Q four pencils A, a, h, and a are to be seen within 

 the margin of the system. This arrangement of light — four coherent 

 ■pencils, arranged in a ^ — must give the image spoken of ; and, 

 especially, the coincidence of A and a must bring out a set of lines, 

 parallel to the axis of the valves, the distance of which is to h as 

 1 : ^3, Those lines are to be seen as contmual ones throiighout the 

 whole valve, if the two pencils a and b are moderated in their brightness, 

 or quite stopped off by a diaphragm above the objective. 



The managing of the system is quite easy. The length of tube held 

 invariably there is no cover-correction necessary, indeed. Extremely 

 thin and extremely thick covers give equal definition. There is only 

 one inconvenience, which till now I did not master. I considered as 

 unquestionable that among the great numbers of fluids, which are fit 

 for use, there would be some, pure or mixed, which would give the exact 

 refractive power of crown-glass and an equal dispersion too. But after 

 having measured the refraction and dispersion of more than sixty oils and 

 other fluids during the last weeks, I find that my assumption was pre- 

 mature ; all the fluids, found till now, which have the refractive index 

 of crown (or are brought to this refraction by mixture with another fluid) 

 give a considerably higher dispersion. Therefore the different thickness 

 of the fluid, caused by the difference of cover, must give a slight altera- 

 tion of the chromatical correction of the lens. 



If I should not succeed in finding a liquid of the right quality, the 

 want will not at all be a grave one. In this case I shall adjust the colour- 

 correction for thin cover, taking the liquid of minimum difference. For 

 a considerably thicker cover, the correction now will be made perfect by 

 adding to the liquid a small portion of a more dispersive fluid. This will 

 not be a great inconvenience, I think. 



In this way I have managed the system till now, taking the oil of 

 copaiva-balsam as base of the mixture and adding varying portions of 

 oil of cloves. Using a small bottle with parallel faces (made of plate- 

 glass) and a small prism of crown-glass plunged in, the refraction and 

 dispersion of the mixture can be adjusted most exactly by simply view- 

 ing the window-bar through the prism and the liquid. 



The provisory setting of the new objective would be somewhat 

 troublesome to you, and besides that I cannot dispense with it in my 

 further experiments with new liquids. From these reasons I have not 

 yet sent it to you ; I prefer to keep it until three new samples will be 

 finished, which are in work now. The best of the four will be brought 

 to your short setting, and will be sent to you as soon as ready, together 

 with the testing-bottle described above. I hope you will agree with this 

 delay. For petrographic work the oil-immersion will be exceedingly 

 convenient ; but besides that, as the greater aperture will yield a greater 

 resolving power, lenses on this plan will be accepted by your English 

 diatomists I think. But they will be much more costly as far as the 

 unusual aperture is wanted — which will not be necessary for petrographic 



April ISth, 1917 P 



