274 Professor Ward [April 27 » 



illumination, &c, but varies if these conditions vary. By comparing 

 a number of sucb curves, moreover, much curious information 

 is coming to hand, but as this does not concern our present theme, 

 I shall not dwell on it here. 



The steepness of the curve — i. e. the rate of growth of the rods — 

 increases with the age and length of the latter, when the conditions 

 of growth are constant, because, as the rod increases in age and 

 length, it exposes a larger and larger surface of absorption to the 

 food-materials, and possesses more and more power of utilising them 

 for further growth. 



If two cultures such as those described are started at the same 

 time, one under one microscope and the other under another by its 

 side ; and if one condition at a time is varied for one of these 

 cultures, then the differences in the curves ought to give us a very 

 clear idea of the effect of the difference of condition on the growth of 

 the bacillus— and we must remember that the rate of growth is our 

 bast criterion for the well or ill doing of the organism. 



It is, however, by no means an easy matter to vary one condition 

 at a time in these cultures, and the principal difficulty of the whole 

 of this more recent part of the research has been the attainment of 

 this object as near as possible. 



The most prominent and annoying form in which this difficulty 

 has presented itself has been the following: — If I allow the sun's 

 rays to be reflected up from the mirror into the culture-cell, in order 

 to brightly illuminate the growing bacillus, the heat-rays at the 

 red-end of the spectrum become effective in raising the temperature 

 of the culture ; whereas the culture on the other microscope, 

 protected from the light, is also sheltered from these heat rays, and 

 consequently the two cultures are not simply differing by one 

 condition — it is not true that the one is growing merely in the dark 

 and the other merely in the light, but the one is growing in the 

 light at a higher temperature than the other. 



This and similar difficulties had to be overcome, and it was clear 

 that some means must be devised for recording the temperature inside 

 the culture cell, and this I accomplished by placing a very small 

 delicate thermometer, with its bulb blackened, inside the culture cell 



[Photograph and description of the control culture cells] 

 itself, so that each time when the register of the length of the bacillus 

 was taken, the temperature of the culture itself was also recorded. 



By these means I have now succeeded in comparing the effect of 

 light of even low intensity by direct observations of the bacillus 

 itself, under high powers of the microscope, and in comparing the 

 curves of growth under various conditions of light and of tempera- 

 ture, and this, moreover, on a Thames bacillus, which is among the 

 least sensitive of those which have yet come under my notice. 



Since the details of this investigation have not yet been pub- 

 lished, however, I must content myself with a few references only. 



Just to illustrate how strikingly different the curves of growth 



