the fact that the optician deems them necessary for 

 determination of optical qualities ought at least to 

 indicate that they are a valuable adjunct and certainly 

 will aid in giving greater manipulative skill. 



At this point it is considered advisable to add some 

 suggestions from Carpenter. 



"The correctness of the conclusions which the 

 microscopist will draw regarding the nature of any 

 object from the visual appearence 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 microscopical observations and 

 upon his knowledge of the class of bodies to which the 

 particular specimen may belong. Not only are observa- 

 tions of any kind liable to certain fallacies arising out 

 of the previous notions which the observer may enter- 

 tain in regard to the constitution of the objects or the 

 nature of the actions to which his attention is directed, 

 but even the most practiced observer 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. The suspension of the 

 judgment, whenever there seems room for doubt, is a 

 lesson inculcated by all those philosophers who have 

 gained the highest repute for practical wisdom, and it 



