10 



Biologists Study Animals and Plants 



Fig. 8 (left) The viodern 

 research ftiicroscope is far 

 different from the siviple in- 

 strument of Leeuwenhoek^s 

 day. (spencer lens co.) 



Fig. 9 (right) One of Leeii- 

 wenhoek's many micro- 

 scopes. A lens was fastened 

 into the metal plate. The 

 rest of the microscope is the 

 object holder, which, by the 

 use of screws, was used to 

 place the object in proper 

 position. Compare this with 

 the moder7i research micro- 

 scope, (bausch and lomb) 



dog. The saliva flowed from the dog's 

 mouth just the same and in the same 

 amounts. This experiment was per- 

 formed many times, and with many dif- 

 ferent dogs. Always the sound of the 

 bell made the flow of saliva start. 



Then Pavlov varied his experiment; in 

 one variation, as he showed the food he 

 touched the dog on its hindquarters 

 instead of ringing a bell; in another 

 variation he showed the food and at the 

 same time showed the dog a paper on 

 which a large circle had been drawn. 

 In each experiment, after enough repe- 

 titions there was a flow of saliva even 

 when the food was withheld. Pavlov 

 showed in this way that not only the 

 normal cause, but also an unusual cause, 

 could lead to the flow of saliva in the 

 dog. The experiments taught scientists 

 something about the way in which 

 animals learn. You will read more about 

 this later in the book. 



But notice how carefully the stage 

 was set for the experiment. Weeks 



spent in training the dog; years of study 

 to make possible the delicate operation; 

 skillful construction of the cages and 

 harness; patient watching for results; 

 accurate measurement and recording of 

 the facts day after day; the repetition 

 of the experiment with many dogs — 

 all this was necessary to make successful 

 w^hat may have seemed to you at first 

 to have been a relatively simple job. 



Biologists study the "invisible." The 

 man who first saw "invisible" or micro- 

 scopic creatures was a Dutchman, Anton 

 van Leeuwenhoek (163 2- 1723), whose 

 hobby was making lenses. When he had 

 ground and polished a small bead of 

 glass until he was sure it would magnif\' 

 well, he used it to examine all kinds of 

 tiny objects to find out what they 

 really looked like. 



It was a great day for him and for 

 biologN' when he examined a drop ot 

 the rain water that had been standing in 

 a barrel. Picture his amazement and de- 

 light when he found that the drop was 



