108 SELECTION AND USE 



struments is that the stages are too small. It should also be 

 firm and substantial, so that its position in regard to the other 

 parts of the stand cannot be varied by slight pressure.* The 

 most important points connected with the stage are the means 

 provided for holding and moving the object, and the facilities 

 afforded for attaching accessory apparatus. 



In the most complete stands, the object is held between 

 sliding clips, which form a sort of clamp that is capable of 

 being moved in two directions, at right angles to each other, 

 by mechanical means, which generally consist of a screw for 

 one direction and a rack and pinion for the other. This form, 

 which is known as the mechanical stage, enables even a com- 

 paratively unskilled person to bring any part of the object into 

 the desired position in the field of view, and this with the 

 utmost precision. These mechanical stages may be said to be 

 characteristic of the higher classes of English microscopes, and 

 as they are expensive, they are not generally used. Neither 

 are they absolutely necessary for ordinary work with low or 

 medium powers, for with any objective lower than one-twelfth 

 of an inch focus, the object can be moved by hand quite as 

 readily as by the screws, and we hold it to be a well established 

 rule in all manipulations connected with scientific work, that 

 whenever any operation can be performed satisfactorily by 

 means of the hands alone, all special contrivances should be 

 dispensed with. For low and moderate powers, therefore, we 

 prefer the plain stage, on which the object is moved by means 

 of the hands alone. But when very high powers are used, and 

 especially when delicate micrometrical or goniometrical meas- 

 urements are to be made, a well-made mechanical stage becomes 

 a necessity. For while it is easy enough to bring an object 

 very near to a given point by means of the fingers alone, it is 

 almost impossible to secure perfect accuracy. In the effort to 

 attain this the mechanical stage is a great assistance, and 

 therefore when Frey utters a wholesale condemnation of the 



*At the same time, however, it must be borne in mind that no stage 

 ever was made so firm that even a slight pressure would not affect if. If, 

 therefore, the reader is determined not to rest content with anything 

 short of a perfectly rigid stage, he will reject all the best microscopes in 

 market. 



