228 IVansactions of the Society. 



Second Lettee. 



Jexa, 

 2th December, 1876. 



Dear Sir, 



The preparation which I announced in my former letter 

 has been sent to your address. I have added one of Mr. Zeiss' 

 aa-objectives, because Mr. Z. was not sure if you would find such 

 a lens at Mr. Baker's shop. I wished to save you the trouble of 

 making the diaphragm, which must be exactly adjusted to the 

 focal length of the system. If you have used the objective, please 

 send it to Mr. Baker. 



The diaphragms herein are marked with numbers : — No. 1 for 

 the duplication of the lines ; No. 2 for showing the disappearance 

 of all the lines if no diffraction pencil is admitted. The broader 

 slit makes disappear only the narrow part of the group, the smaller 

 slit both parts. 



The same diaphragms serve for showing the secondary lines on 

 the crossed gratings — one set of lines after the other. The broader 

 slit fits to the fine groups. No. 3 gives hexagonal markings, alike 

 to those of Pleuro angulatum on the group [in which the lines are 

 inclined at 60°], and quadratical markings on the group [lines 

 crossing at right angles]— the wide aperture for the finer groups. 

 This effect depends on the stopping-off of all the diffraction pencils 

 except those which are next to the incident ray. With P. angu- 

 latum this condition is fulfilled without any artificial help in any 

 lens, as great as the aperture may be- — in consequence of the great 

 dispersion of the diffraction pencils. 



From this point of view the experiment named is of some 

 interest. It proves that the markings on that diatom must not 

 necessarily be the image of a real structure of the same kind : 

 those markings have their sufficient reason in the admittance of 

 three (or six) diftraction pencils with exclusion of any other. 



No. ,4 produces the same hexagonal markings on the finer 

 group [of crossed lines] — but only in three distinct positions of 

 the three holes, relating to the lines — only in those positions, in 

 which the three holes coincide with three pejicils of the diffraction- 

 spectrum. 



Making the experiments on the crossed lines with a narrow 

 beam of light (from a distant flame) in order to see the spectra 

 separated, is somewhat difficult from want of light, by which deep 

 eye-pieces are excluded. All the experiments are much more 

 striking, and make no trouble at all, if you resign to see the 

 spectra and to compare their position with the position of the 

 holes. You will use now a broad beam of light, filling the apeiture 

 of the objective. Though the spectra are superposed in this case 

 and tliough direct rays will pass through all the apertures in every 



