Otf THE MICROSCOPE. 237 



easily register the location of any object of interest, and so be 

 certain of finding it at any future time. 



The mechanical stage, or the ordinary movable glass stage, 

 described at page 110, or that shown in Fig. 24 (page 111) af- 

 fords special facilities for using the Maltwood Finder. But un- 

 less fitted with some special contrivance, microscopes which 

 have only the ordinary 

 clips, are unsuitable for 

 this purpose. A movable 

 ledge and stop, which 

 may be used with the 

 common spring clips, is, 

 however, easily devised 



and made. We often use 8o._ 8TO P FOR MALTWOOD FIN-DEI-, 



a simple wooden slide 



with a little projection on the edge near one end. The wooden 

 slide must be thicker than the glass slide which carries the 

 object to be registered, as it is necessary that the springs should 

 hold the wooden slide firmly in place, while the object-slide 

 moves freely below the springs. A piece of vulcanite or hard 

 rubber about the eighth of an inch thick and the size of the 

 usual 3x1 glass slide answers admirably. It should have a 

 stop fastened to one end, as shown in figure 80, where a is the 

 hard rubber slide, and b is a small piece of brass, let into the 

 end and screwed fast. 



Microscopical Misinterpretations. The observer 

 who uses the compound microscope labors under certain dis- 

 advantages which do not affect those who examine large ob- 

 jects which can be handled, and thus subjected to the scrutiny 

 of several senses. The fallacies to which the microscopist is 

 liable in this way deserve special attention and special precau- 

 tions. We have already (page 198) alluded to certain fallacies 

 of another kind, which must be avoided by such careful and 

 extensive study as will enable us to recognize foreign matter 

 when we see it; the fallacies now under consideration can only 

 be avoided by careful study of the laws of optics, and by intro- 

 ducing considerable variety of methods into our examinations. 

 One of these sources of fallacy arises from the liability which 

 most persons have to see .objects pseudoscopicctlly, as it is called 

 that is to say. hollows appear to be elevations, and elevations 

 appear to be hollows. The extent to which this tendency ex- 

 ists is not generally recognized. Taking a gold coin, on which 

 the letters, etc., were known to be raised, we placed it under 

 the microscope, and submitted it to seven intelligent persons. 

 Out of these, five declared that they saw the letters sunk into 



