122 THE FROG CHAP. 



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 corpitscles. 



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 -fafa 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 lower 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 numerous, having 

 a granular appearance, and an irregular or rounded outline. 



Focus a colourless corpuscle under the high power, and note its 

 amoeboid 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. Eketch. 



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 corpuscle becomes transparent, 

 while the nucleus is rendered distinct. 



