1894.] on the Action of Light on Bacteria and Fungi. 273 



A glass tube about 3 inches long, has a bulge in the middle : this 

 bulge is ground down on two opposite sides so that it is pierced by- 

 two large round openings. One of these openings is placed down- 

 wards on a piece of glass or quartz, and cemented to the latter by 

 any convenient means — I usually employ paraffin : the other opening, 

 above, is covered by a thin piece of glass, to the under side of which, 

 in the centre, is hung a minute drop of the food-material containing 

 the spore to be examined. The two arms of this culture chamber are 

 stuffed with wet cotton wool, and a layer of water is placed on the 

 floor of this "damp chamber," and the whole is now ready for use. 

 All the materials employed have undergone the necessary cleaning and 

 sterilising, and with proper precautions there is no reason why such 

 a hanging drop, with the growing organism in it, should not be 

 kept under observation until the latter has either exhausted all 

 the food supplies, or completed its life-cycle, in focus under the 

 microscope. 



When in position on the stage of the microscope, the light reaches 

 the organism through the glass or quartz, and of course it is easy to 

 arrange shutters, screens, &c, so that only the light reflected from 

 the mirror of the microscope below shall reach the drop and the 

 organism in it. Again, it is easy to interpose coloured or other 

 Bcreens of glass, coloured solutions, &c, and so allow only certain 

 rays to reach the hanging drop. 



Now suppose a single spore in the drop to be placed in focus 

 under the microscope, and that in the eye-piece of the latter a 

 graduated scale is fixed, the value of whose divisions is known. In 

 a few hours the spore, the length of which is measured, begins to 

 germinate, and developes into a rod — a bacillus — the length of which 

 is then measured ; then measurements of the increasing length of the 

 growing rod are taken at successive intervals, as convenient, and so 

 ideas of the rate of growth of the rod obtained. If I sketch the 

 elongating rod at appropriate intervals on sectional paper, to scale, 

 always starting each sketch from the same horizontal base-line, and 

 at a distance along this line proportional to the time which has 

 elapsed since the preceding sketch was made, the line joining the 

 free ends of the rods gives a curve, which will be steeper or the 

 reverse according to the rapidity of growth of the rod. Considerable 

 use can be made of this curve of growth of the bacillus. 



[Demonstration of the measurement of growth of the bacillus.] 



Obviously it is easy to record these measurements by plotting 

 them out in the form of a curve — the intervals of time being 

 measured along a base line, and vertical lines (ordinates) in each 

 case proportional to the length of the growing rod at the time, being 

 erected at these points, and the joined upper ends of these ordinates 

 give the curve. 



[Photograph of a curve of growth of a bacillus.] 



Now I find that the curve of growth thus obtained is remark- 

 ably constant in shape under constant conditions of temperature, 



