CHAPTER V 

 MAGNIFICATION AND MICROMETRY 



225. Apparatus and material for Chapter V. 



1. Simple and compound micro- 5. Stage micrometer ( 233). 

 scope (228). 6. Wollaston camera lucida ( 234). 



2. Block for magnifier and com- 7. Ocular screw micrometers and 

 pound microscope ( 230, 236). fixed ocular micrometers ( 238, 241). 



3. Steel scale or rule divided into 8. Necturus red blood corpuscles 

 millimeters and I mm. ( 231). ( 248). 



4. Dividers ( 231). 9. Eikonometer ( 253). 



WHY A MAGNIFIED IMAGE is NECESSARY 



226. The fundamental reason for using a microscope lies in the 

 structure of the eye and its possibilities of adjustment for objects 

 at different distances. 



The sensory receptors or neuro-epithelium (rods and cones) of the 

 eye stand in general with their long axes parallel with the rays of light 

 entering the eye, hence the image of any external object falls ^on the 

 ends of the sensory receptors. Now it is believed that if any image 

 falls wholly upon one of the receptors it will appear as a point, and if 

 the image of two objects close together were to fall on one receptor 

 the two objects would appear as one. 



227. Robert Hooke (1674), in dealing with the power of the hu- 

 man eye to distinguish double stars and to see two points or two 

 details of an object as two, concluded that the two stars or the two 

 points of any object must at be least far enough apart to make the 

 visual angle one minute. A few people can distinguish double stars 

 with a visual angle less than one minute, but for many people the 

 visual angle must be greater. If the visual angle is too small, then 

 the two stars or two points appear to fuse and form one. The visual 

 angle of one minute then does not represent the limit of visibility, but 

 the limit of resolution, that is, seeing two objects as two separate things, 



127 



