52 THE MICROSCOPIST. 



and using the fine adjustment the best position will be 

 shown by the sharper and more delicate image of the 

 object. 



For other details respecting adjustment the reader is 

 referred to the chapter on Microscopic Accessories. 



Errors of Interpretation. True science is hindered most 

 of all by speculation and false philosophy, which often 

 assume its garb and name, but it is also retarded by im- 

 perfect or false observation. It is much less easy to see 

 than beginners imagine, and still less easy to know what 

 we see. The latter sometimes requires an intellect of sur- 

 passing endowments. The sources of error are numerous, 

 but some require special caution, and to these we now 

 refer. 



The nature of microscopic images causes error from 

 imperfect focal adjustment. We see distinctly only that 

 stratum of an object which lies directly in focus, and it is 

 seldom that all parts of an object can be in focus together. 

 Hence we only recognize at once the outline of an object, 

 but not its thickness, and, as the parts which are out of 

 focus are indistinct, we may readily fall into error. Glasses 

 vary much in this respect. Some have considerable pene- 

 trating and defining power even with moderate angular 

 aperture, and are better for general work than those more 

 perfect instruments which give paler images and only re- 

 veal their excellencies to the practiced microscopist. 



Sometimes the focal adjustment leads to error on ac- 

 count of the reversal of the lights and shadows at differ- 

 ent distances. Thus the centres of the biconcave blood- 

 disks appear dark when in focus, and bright when a little 

 within the focus; while the hexagonal elevations of a dia- 

 tom, as the Pleurosigma angulatum^vQ light when in focus, 

 with dark partitions, and dark when just beyond the 

 focus. From this we gather a means of discrimination, 

 since a convex body appears lighter by raising the micro- 

 scope, and a concave by lowering it. 



