598 



Journal of Applied Microscopy. 



THE LICHENS. 



Some of these forms may be cut in paraffin, but most of them are so hard or 

 leathery that better results may be secured with celloidin. All of the lichens 

 should be fresh and moist when they are put into the fixing agent. Wherever 

 difficulty is anticipated in the cutting, picro-acetic acid may be recommended for 

 fixing and usually, celloidin for imbedding. The younger apothecia of forms 

 like rJiyscia, Usnea, Sficfa, and J'elfigera cut well in paraffin, but results are 

 uncertain with the older apothecia. 



( To he coiitiinted.) 



Bacterial Measurements. 



The method here described has been in use in this laboratory for the past 

 two years, and has been found a simple, accurate, and satisfactory means of 

 measuring bacteria and of recording those measurements. 



The photographic apparatus is so adjusted that an amplification of one 

 thousand diameters is secured. This is determined by measuring the magnified 

 image of a stage mici'ometer on the ground glass screen. 



A drawing is made of a convenient size, by ruling, with ink, two sets of 



^^ 



equidistant lines at right angles to each other, each tenth line being somewhat 

 heavier than the others ; this drawing should be at least four times the size of 

 the negative to be prepared from it, in order to secure the requisite fineness of 

 the lines. This drawing is then reduced by photography to such a size that 

 the rulings are exactly one millimeter apart. The negative so obtained is the 

 scale used. 



The image of this scale is superimposed on the image of the photo-micro- 

 graphic negative by a process of double printing, the photo-micrographic nega- 

 tive being printed first, and the scale afterwards on the same paper, or %nce 

 versa. 



