62 



THE MICROSCOPE 



Culture 

 plates. 



chess-board plate be used, then thirty-six squares, six each way, 

 correspond to a cubic millimetre. The most convenient size to 

 select wiU depend upon the class of object to be counted and the 

 object glass that is used. 



Due to the alternate squares being tinted, a count can be made 

 with much less eye-strain than with the ordinary hsemacytometer, 

 and this method is preferred by some apart from the question of 

 the cost of the apparatus. 



The preparation of culture plates and the methods of cultiva- 

 tion will be found in text-books on bacteriology. They are large 

 square plates covered on one surface with nutrient gelatine, upon 

 which isolated colonies of bacteria are groAsdng. They should 

 be examined with a low-power IJ-inch (32-mm.) object glass, 

 the mechanical stage having been removed from the surface of 

 the stage for the purpose. The required colonies having been 

 recognised, a morsel of the gelatine can be removed with a 

 platinum needle, while the colony is in the field of the microscope, 

 and can be smeared on a cover glass. A drop of distilled water 

 having been added, it can be spread out on the cover glass and 

 examined in a living state with the high-power dark-ground 

 illuminator or dried in a spirit flame, stained and moimted on 

 a 3 X 1-inch slip with a drop of Canada balsam. 

 Warm stage- A Warm stage is an apparatus for applying warmth to a speci- 

 men under continuous observation. A simple form consists 

 of an oblong copper plate 3x1 inches, 

 from one side of which projects a long 

 narrow strip and which has an aperture 

 1/2 inch diameter in the centre of the 

 3 X 1-inch portion. It is placed on the 

 stage of the microscope and held like an 

 ordinary 3x1 glass slip in such a posi- 

 tion that the long strip projects in front 

 of the microscope. A spirit lamp is placed 

 under the far end of the projecting strip 

 and adjusted so that its flame impinges 

 on the strip, or is slightly to one side, 

 until the portion of the copper plate 

 which is near the 1/2-inch aperture is at 

 blood heat. The correct temperature is 

 readily ascertained if a small piece of a 

 mixture of cacao butter and wax is placed 

 on the copper near the aperture. The 

 mixture is made in such proportions that 

 it melts at blood heat, and when the piece melts on the copper 

 the correct temperature has been reached. 



The drop of fluid to be examined is placed on a large cover 

 glass and a smaller cover glass is placed over it, and the two laid 

 upon the copper plate. To prevent evaporation the upper 



Fig. d5.— No. 3384, 

 Warm Stage. 



