1 64 INTRODUCTION TO GENERAL PHYSIOLOGY 



of the rays at a few inches distance from the first, we bring the 

 rays to a focus at a short distance away. A real image is formed 

 here, as may be seen by receiving it on ground glass. This lens 

 corresponds to the field lens of the ocular of the microscope, and 

 this lens may be used for the experiment. The eye-lens has the 

 function of magnifying the above image, just as an ordinary pocket- 

 lens does. In this case the image is not real, the eye being placed 

 close to the lens in the emergent beam. The image on the retina 

 in the eye is produced by the refractive system of the eye itself. 



A graduated scale for measuring objects, or ruled in squares 

 for counting their number, may be placed at the position of the 

 focus of the field lens, and thus be seen superposed on the object. 



Another form of ocular is sometimes used. In this, no real 

 image is formed : the eye-lens being closer to the field lens than 

 the position of the focus of the latter. 



In making experiments on the properties of lenses, they may 

 be held in retort stand clamps. 



The other details of the microscope stand can best be studied 

 on the instrument itself. Below the stage on which the object is 

 placed, we have arrangements for sending light through this. The 

 tube containing the observing lenses is provided with means for 

 accurately and conveniently bringing it to the correct distance from 

 the object in order that the image may be formed in the ocular at 

 the right place. Various objectives and oculars of different focal 

 distance or magnifying power can be placed in position in the tube. 



An appropriate microscope is that sold for bacteriological 

 purposes, and may be obtained from several British makers. It 

 should possess a sub-stage condenser for illumination with high 

 power objectives, and also a dark-ground condenser to fit into the 

 same tube as the ordinary one. Objectives of 25 mm. and 4 mm. 

 focus are required, and a 2 mm. oil immersion is desirable. One of 

 the oculars should have a micrometer scale. 



Nature of Protoplasm. One of the protozoa, Amccba, is the 

 best object for this study. As Professor Graham Kerr points out in 

 Nature (3ist October 1918, p. 166), the species required is the large 

 form, which is to be found in water containing abundant food 

 material and oxygen. Such a situation is the water trickling from 

 a boggy spot. 



If masses of weeds be collected from a pond and allowed to 

 putrefy in water in a number of shallow covered dishes, amoebic 

 can often be found in the slime scraped from the surface of the 

 plants in some of the dishes. They are said to be most abundant 

 in about a fortnight, and then commence to disappear. 



Another way is to collect the upper layer of the ooze from the 

 bottom of a shallow ditch or pond and allow it to settle in tall 



