THE SCIENCE OF MICROSCOPY. 3II 



principles which, after trial and verdict by competent authority, 

 have become incorporated in the general aggregate of accepted 

 scientific truths. But it will be well if he study to reap the benefit 

 of these principles in his own practice. He need not constitute 

 himself a pioneer over unknown or disputed ground, but it will be 

 well if he hold himself ready to follow each onward step, and try 

 to gain clearer conceptions of the powers and limits of his 

 instrument. And until he thus accept the full significance of 

 theoretical teaching, and avail himself of its aid, he can scarcely be 

 said to realise the importance of a truly scientific method. Nor 

 can it be wondered that error of observation should be so frequent, 

 so long undetected, so diflicult to rectify, or that appearances 

 suggesting false views of the structures they were believed to 

 represent, should neither be suspected nor detected. 



But apart from any application of optical science, the accuracy of 

 observations made by the practical microscopist depends upon his 

 individual qualifications and experience. And it may be urged that 

 good natural power of sight, aided by training, should ensure a 

 correct mental grasp of what is seen, more certainly than theoretical 

 knowledge. As this point will be discussed a few pages further on, 

 I will here only observe that good sight gives its possessor great 

 advantage, but does not necessarily confer any power of discrimin- 

 ation where judgment depends not upon the clearness, but on the 

 correctness of the image, i.e., upon our inference from what is 

 most clearly seen. And again, training may suffice for such 

 management of the instrument and illumination of object as will 

 enable the observer to '' bring out " well known appearances, but 

 it will not explain how identical images may be got out of non- 

 identical objects, or why the same object shall yield different 

 images ; nor will it supply safe conclusions from the known to the 

 unknown, unless good sight be supported by good reasoning. 

 Admitting fully the advantage of good sight and the necessity of 

 scientiJiG training (there is such a thing as had training !) it doss not 

 follow that theoretical knowledge is therefore less wanted, or that 



