27 



times, in my laboratory, scarcely varied three degrees in a 

 week. 



To observe Bacteria under the microscope it is best 

 to have a good one-eighth dry lens, which will shew all the 

 larger ones fairly well. But for the smaller varieties it is 

 absolutely necessary to have a high power. There are several 

 of these on the table, lent by Messrs. Beck, and the best of 

 them is their one-tenth water immersion, which defines very 

 clearly. 



The lens to my own microscope is a one-twelfth oil 

 immersion made by Powell and Lealand. It was used at 

 the Biological Laboratory at the Health Exhibition for all 

 the more important investigations which were carried on in it, 

 and was the best there. 



There must also be an achromatic condenser affixed to 

 the substage. This is merely a lens turned upside down to 

 concentrate the rays of light from the mirror on the lower 

 surface of the objects. I have had one fitted to my micro- 

 scope b}' Messrs. Beck, but you may see that Dr. Tayler 

 has made one for himself, with an ordinary object glass, 

 which answers the purpose admirably. 



To examine micro-organisms in the fresh state, a 

 minute portion from a cultivation may be placed on a 

 slide, with the point of a platinum needle, a drop of steri- 

 lised distilled water being added, if necessary, and covered 

 with a clean cover glass. They look like glittering moving 

 particles, and are best seen through a small diaphragm, 

 admitting but little light. 



The most satisfactory way, however, of preparing 

 Bacteria for the microscope is by staining, for they will take 

 the colouring matter of certain aniline dyes, whilst fatty 

 and albuminous granules, which may be mistaken for them, 

 will not do so. A little of the specimen is placed on a cover 

 glass, with a small drop of sterilised distilled water, and 

 another cover glass is placed upon it. The two glasses are 

 then moved upon one another, between the finger and thumb, 

 until there is a well distributed film on each. They are then 



