122 THE FROG CHAP. 



coagulating before it is covered, in which case it will not 

 spread out into a transparent layer : if the cover is lowered 

 too suddenly, bubbles of air are commonly included. 



Examine your preparation, first of all, under the low 

 power, and learn to recognise the appearance of air -bubbles. 

 Note the colourless plasma and the numerous minute blood- 

 cells or corpuscles. 



Now replace the low by the high power. Bear in mind 

 that the higher the power, the shorter the focal distance. 

 With the low power you will probably find that the object 

 is in focus when about half an inch from the bottom lens of 

 the objective. The high power, on the other hand, has to 

 be brought to within about ^th of an inch of the cover- 

 glass, which is therefore liable" to be broken and the lens to 

 be injured by careless focussing. The safest plan is to screw 

 down the tube, keeping your eye at the level of the stage, 

 until the objective almost touches the cover-glass : then, 

 looking through the microscope, very slowly raise the tube 

 by means of the coarse adjustment, until the object comes 

 into view. Note (Fig. 26) : 



a. The numerous flat, oval, red corpuscles, each with a 

 central swelling in which the nucleus is contained. -Sketch. 



b. The colourless corpuscles or leucocytes, much less numer- 

 ous, having a granular appearance, and an irregular or 

 rounded outline. 



Focus a colourless corpuscle under the high power, and 

 note its amceboid movements : sketch its outline rapidly but 

 accurately, and after a minute or two make another sketch, 

 and then another, until some half-dozen outline drawings 

 of the corpuscle have been obtained : then compare your 

 sketches. 



Now place on the slide, against one edge of the cover -glass, 

 a drop of methyl-green, and against the opposite edge a 

 small strip of blotting paper. If the blood is sufficiently 

 fresh and if it has coagulated you must get another drop 

 the blotting paper will slowly absorb the blood on one side, 

 and the methyl green will be drawn in and will gradually 

 mingle with the blood. When this has taken place, put a 

 drop of salt solution in the place of the methyl-green, and 

 allow it to be drawn across so as to remove the superfluous 

 stain : then remove the blotting paper, and examine the 

 blood once more under the high power . Notice the nucleus 

 present in each kind of corpuscle, and the surrounding 

 protoplasm. Sketch. 



To a fresh drop of blood add, in the same manner, a drop 

 of I per cent, acetic acid. Note that the body of each cor- 



