ZOOLOGY AND BOTANY, MICKOSCOPY, ETC. 257 



(say three minutes) a distinct accumulation of bacteria in the ultra-red 

 was visible. 



Although the changes in the sensitiveness of the reagent and other 

 circumstances prevented accurate measurements in all cases, those 

 cases are of special importance in which the insertion of the absorb- 

 ing medium had no effect on the photo-kinetic reaction. For the 

 evident reaction in both cases with the same minimum width of slit 

 shows conclusively that the medium experimented on is highly 

 permeable for the group of rays in question. 



Absolute diathermancy for the inner ultra-red rays between • 8 

 and • 9 /x wave-length was determined in this way for all colourless 

 transparent media, e.g. glass, water, concentrated solution of alum 

 (these three in layers up to a thickness of 10 cm.), crystals of alum 

 (0-3 cm. thick), the aqueous humour, vitreous humour, crystalline 

 lens, and cornea of the eye. Also some coloured media, as alcoholic 

 solution of chlorophyll (in transmitted light red and nearly opaque), 

 and to a considerable extent thin light-green ivy leaves, and blue 

 moderately dark cobalt-glass. In all these the outer visible red 

 rays also passed through readily. Green and blue-green glass, 

 although comparatively very transparent, caused great weakening of 

 the ultra-red group ; and they also greatly weakened the outer visible 

 red rays. 



A special interest was attached to the experiments on the media 

 belonging to the eye, as affording a contribution to the solution of the 

 question as to the cause of the invisibility of the ultra-red rays. 

 Previous experiments, especially those of Franz and Tyndall, have 

 sufficiently shown that the cause lies in the want of sensitiveness of the 

 elements of the retina, not in the impermeability of the media of the 

 eye for " dark heat." But it is still of great interest for this to be 

 proved for the outermost visible rays nearest to the ultra-red, which 

 have not hitherto been isolated, or at least not in the limited space 

 and sharp definition which can be effected by the photometric bac- 

 terium. Since the experiments with it show the apparently perfect 

 permeability of all the media of the eye for this group of rays, they 

 prove that the limit of visibility of the spectrum at the red end is 

 also the limit for the sensitiveness of the elements of the retiua for 

 less refrangible rays. 



In these experiments the microspectrum was projected into the 

 drop by means of one of Zeiss's lower objectives, B or C. The 

 source of light was the flame of a large Sugg's gas-burner, placed at a 

 distance of 1 in. from the mirror of the microspectral apparatus. 

 The Microscope was placed in a dark box. Under these conditions, 

 when the rays fell directly a very distinct accumulation of the bacteria 

 in the inner ultra-red took place, even with a width of slit of from 

 • 01-0 • 02 mm., although the intensity of the light was then so small 

 that the detection of the bacteria became somewhat difficult even in 

 the bright part of the spectrum with very powerful magnifying. For 

 this reason a somewhat low-power objective C, and eye-piece No. 3, 

 were used. 



A glass vessel with flat parallel walls 3 • 5 cm. wide and high, and 



Ser. 2.— Vol. III. S 



