OP THE MICROSCOPE. 109 



English microscopes, and asserts that they are unpleasantly 

 loaded with what he is pleased to call "screws and unessential 

 appurtenances," it seems to us that he commits a great error. 

 These costly and complex instruments are intended for the 

 highest class of work, and the most powerful objectives; per- 

 fection of the work to be done, and not simplicity in the means 

 by which it is to be done, is the end sought, and this can be 

 attained only by the complex means employed. 



We have never found any of the so-called lever stages that 

 fulfilled the requirements of the highest class of work, and, 

 therefore, if a mechanical stage is to be chosen at all, the best 

 form should be procured. 



A microscope fitted with a good mechanical stage leaves 

 nothing to be desired, but when other forms are used, it is evi- 

 dent that the chief points to be attained are these: 1. The 

 object should be held steadily, but at the same time perfect 

 freedom of motion should be allowed. 2. It should be possi- 

 ble to remove instantly from the surface of the stage, every- 

 thing in the shape of clips and holders, so that a clear field 

 should be left for the adjustment of very large slides, plates, 

 etc., or for the rotation of the object in relation to the light. 

 3. Even the simplest forms of the stage should be so constructed 

 that it may be possible to pass every part of the object under 

 the field of view, and this, without any risk of omitting even 

 the smallest portion. This point is of special importance to 

 physicians and naturalists. Thus, it not unfrequently happens 

 that it is desirable to know whether or not certain forms are 

 present in a given drop of liquid; unless we can subject every 

 part of that drop to microscopical examination, we cannot be 

 sure that the forms we are looking for are absent. There is 

 always a risk of omitting some portion of the slide, and conse- 

 quently doubt must always hang over the exhaustiveness of all 

 our examinations. The only certain means of avoiding all risk 

 <f missing any portion of a given slide is to pass it across the 

 field of view in successive parallel bands, just as a plowman 

 plows a field. The process is clearly shown in the diagrams 

 on the following page, Fig. 22, showing the mode in which the 

 entire surface is completely covered with a series of parallel rib- 

 bons, the breadth of each of which is the diameter of the field 



