ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 30 1 



Photographs of the ultra-violet spectra, beyond 185 /*, of the fol- 

 lowing substances were made : — Nitrogen ; Oxygen ; Water ; Carbon 

 monoxide ; Carbon dioxide ; Hydrogen. 



Nitrogen. — Emission spectrum : beyond 185 //. no bands. Absorp- 

 tion spectrum : very transparent, even beyond 162 /*, but absorbed par- 

 ticular wave-lengths energetically. 



Oxygen. — Emission spectrum : three continuous maxima at about 

 185 /a. Absorption spectrum : rays absorbed in the neighbourhood of 

 185 /a in a series of well-defined groups, fourteen in number. 



Hgdrogen. — -The author states that none of the spectra beyond 185 //. 

 with which he is acquainted emits such a wealth of lines or extends so 

 far as this. Hydrogen reaches its highest photographic efficiency at 

 162 jjl, and extends to approximately 120 /x ; but as such wave-lengths 

 have not yet been measured, it is impossible to give the exact limit. A 

 single plate of fluor-spar at the lower limit reduced the photographic 

 effect to one-half. Hydrogen is extremely transparent. 



Air. — The absorption effect of strata of the following thicknesses 

 was tried : 15, 14, 4, 3, 2, 1, ' 5, • 25, ' 1 mm. With the same time of 

 exposure and same width of slit, the greater thicknesses stopped the 

 spectrum entirely at 178 yu., but a stratum of 4 mm. allowed the first 

 band at 170 /x to appear. With a stratum of ' 5 mm. the spectrum runs 

 to the end of the plate, corresponding to 163 jjl, and with the lesser 

 thicknesses of air still further. Attention is drawn to the profound 

 effect caused by introducing such a thin layer of air at atmospheric 

 pressure into the path of the rays. 



The paper is throughout illustrated by photographs of spectra. 



In connection with V. Schaudinn's interesting experiments, J. W. 

 Oifford observes that, with fluor-spar and the melted quartz now 

 available, there is no doubt that object-glasses could be made, and a 

 vacuum Microscope constructed for use with these very remote rays. 

 In which case, the present resolving power of the Microscope (other 

 things being equal) would be increased, roughly speaking, sevenfold. 

 This means that objects could be separated when the interval between 

 them was no greater than one nine-hundred-thousandth of an inch = 



•02-S2;U. 



Note on the Diffraction Theory of the Microscope as applied to 

 the Case when the Object is in Motion.* — According to the Abbe 

 theory of microscopic vision, says R. T. Glazebrook, when a grating is 

 placed on the stage of a Microscope and illuminated by plane-waves, 

 diffraction images are formed in the focal-plane of the object-glass, 

 and the images in the view-plane result from these — and this is un- 

 doubtedly true. The following difficulty has, however, been raised : 

 if the grating be moved in its own plane in a direction perpendicular 

 to the ruling, the diffraction images do not change ; those seen in the 

 view-plane move : how then can the latter images be due to the 

 former ? The answer lies in the fact that in the above argument the 

 effect of the differences of phase among the diffracted images has been 

 neglected. The diffracted images are not all in the same phase, their 



* Proc. Physical goo., 1904, p. 162. 



