124 



name given to the physiological recognition of about 460 millions 



of millions of taps on the retina in a second of time; whereas 



if the number of such vibratory shocks increased to about 678 



millions of millions, we called the light violet ; the photographic 



plate, however, "cared not for these things," and only showed 



contrast as an indication of different exhibitions of light intensity; 



at Least, that was what it practically amounted to. Hence, if the 



photographer wanted to increase contrast when photographing 



stained bacteria — say a blue bacillus on a white ground — he must 



reduce the intensity of the blue to make it become as dark as 



possible. Dr. Spitta then showed how this could be done by 



using a certain yellow screen which turned the image of the 



blue bacillus black, the field becoming of the same yellow as the 



screen. If, then, a photographic plate was chosen which was 



sensitive to the same yellow tint as the screen, a photograph 



could easily be taken. Illustrations were given by means of the 



lantern. Dr. Spitta next showed how necessary it was, after 



what he had just said, to know the sensitive properties of most 



kinds of orthochromatic plates on the market, the peculiarities 



of each being designated by him as its " eye." A lantern slide 



was here thrown on the screen, showing the audience the " eye " 



of most of the orthochromatic plates to be procured, using both 



short and long exposures (Plate 8). It was very instructive, 



f < »r it showed at what wave-length the sensitiveness commenced 



and finished for all varieties of emulsion. Step by step the 



audience were taken through the subject, and shown in detail 



how they might study these peculiarities for themselves. Dr. 



Spitta also exhibited and briefly described the form of spectro- 



scope which he had employed in his researches. It was made by 



Zeiss, and its special feature lay in the projection, by means of 



a scale, of the wave-lengths of all the different colours in the 



spectrum upon the plate, at the same time as the spectrum was 



being photographed. This was a great convenience, and placed 



the method of comparison upon a scientific basis, for it had often 



been pointed out how useless remarks were, when employed by a 



lecturer who spoke of effects "somewhere in the blue " or "near 



the <-nd of the yellow," instead of in terms of the wave-length of 



(he colour at the situation to which reference was made. Real 



spectra were now thrown on the screen by means of a second 



lantern litted with a large bisulphide prism. These, while 



