EERORS OF INTERPRETATION 427 



the coarse adjustment ; only tin- very finest details, sucli as the podura 

 'exclamation' marks, require the fine adjustment. 



]>evond the correct and judicious use of the microscope and all 

 its appliances, there is the matter of the elimination of errors f>f in- 

 terpretation to be carefully considered. 



The correctness of the conclusions which the microscopist will 

 draw regarding the nature of any object from the visual appearances 

 which it presents to him when examined in the various modes now 

 specified, will necessarily depend in a great degree upon his previous 

 experience in microscopic observation and upon his knowledge of 

 the class of bodies to which the particular specimen may belong. 

 Not only are observations of an;/ kind liable to certain fallacies 

 arising out of the previous notions which the observer may entertain 

 in regard to the constitution of the objects or the nature of the actions 

 to which his attention is directed, but even the most practised ob- 

 server is apt to take no note of such phenomena as his mind is not 

 prepared to appreciate. Errors and imperfections of this kind can 

 only be corrected, it is obvious, by general advance in scientific 

 knowledge: but the history of them affords a useful warning against 

 hasty conclusions drawn from a too cursory examination. If the 

 history of almost nn// scientific investigation were fully made known, 

 it woidd generally appear that the stability and completeness of the 

 conclusions filially arrived at had only been attained after many 

 modifications, or even entire alterations, of doctrine. And it is 

 therefore of such great importance as to be almost essential to the 

 correctness of our conclusions that they should not be finally formed 

 and announced until they have been tested in every conceivable 

 mode. It is due to science that it should be burdened with as few 

 false facts and false doctrines as possible. It is due to other truth- 

 seekers that they should not be misled, to the great waste of their 

 time and pains, by our errors. And it is due to ourselves that we 

 should not commit our reputation to the chance of impairment by 

 the premature formation and publication of conclusions which may 

 be at once reversed by other observers better informed than our- 

 selves, or may be proved to be fallacious at some future time, per- 

 haps even by our own more extended and careful researches. The 

 suspension of the judgment /'//'/ wv tlcre ,sw;//* room for doubt is 

 a lesson inculcated by all those philosophers who have gained the 

 highest repute for practical wisdom ; and it is one which the micro- 

 scopist cannot too soon learn or too constantly practise. I Jesides t hese 

 general warnings, however, certain special cautions should be given 

 to the young microscopist with regard to errors into which he is 

 liable to be led even when the very best instruments are employed. 



Errors of interpretation arising from the imperfection of the 

 focal adjustment are not at all uncommon amongst microscopists, 

 and some of the most serious arise from the use of small cones of 

 illumination. With lenses of high power, ami especially with those 

 of large numerical aperture, it very seldom happens that all the 

 parts of an object, however minute and flat it may be, can be in 

 focus together; and hence, when the focal adjustment is exactly 

 made for one part, everything that is not in exact focus is not only 



