Report on the Bacteriology of Water. 333 



Before leaving this part of the subject, there is one other criti- 

 cism to be taken into account. Experiments show that great care 

 must be exercised with respect to the cooling effects of radiation 

 from the thin coverslips supporting the hanging drops. I shall have 

 occasion to illustrate this as we proceed ; it is not so much that one 

 does not recognise a danger of this kind, as that one does not know 

 the magnitude of its effects until experiments have been made to 

 determine them, that makes it worth while to note it particularly. 



As we shall see, the danger does not depend entirely on the direct 

 effects of the fall of temperature on the organism in the drop, but 

 also on the dilution of the drop by condensation of water, and some- 

 times making it spread over the surface of the glass, and so on. 

 These, and difficulties of similar kind, have given much trouble, and 

 sometimes resulted in the ruin of experiments which promised useful 

 results. 



Measurements of Growth in Light and Dark, under Known Conditions 

 as to Temperature. 



I now proceeded to make experimental cultures with due regard to 

 the information previously obtained, and especially taking care to 

 check the temperatures by keeping control cells which I now have 

 expressly made with small, black-bulb thermometers in them. More- 

 over, as time went on, and experience increased, I discarded the use 

 of glass as much as possible, and eventually succeeded in getting rid 

 of it altogether; for the whole tendency of experience in these 

 researches has been to show that the more glass the light rays have 

 to traverse, the more difficult it is to trace their effect. 



At first, however, I had to be content with using open windows and 

 quartz floors to the cells, the light rays being reflected from the glass 

 mirror of the microscope and passed through glass or other screens, 

 &c. Subsequent developments will be described as I proceed. 



In the first set of examples of experiments it will be noted that the 

 exposures are from the beginning i.e., it is the spores themselves 

 which are submitted to the light-action ; subsequently, I give experi- 

 ments where the spores are first allowed to germinate out, and it is 

 the actively growing rods or filaments which are exposed anring post- 

 germinal life. 



Experiments with Spores. 



Spores were sown as usual at 9 A.M., in broth drops, in cells with 

 quartz floors, and the two cultures kept in the dark at 15 C. while 

 further preparations proceeded. 



By 10.15 A.M. four microscopes were fitted each with two flat 



