148 PROCEEDIIS'GS OF THE AMERICAN ACADEMY 



think, to refute these views ; but I would like to say a few words more 

 in conclusion, especially in reference to the general relations of micro- 

 scopical investigations to other departments of natural history. 



" To say that objectives with a wide angle of aperture and a flat 

 field, are needed for only a few bodies, such as test objects, like the 

 Diatomaceae and other known difficult subjects, is to ignore the whole 

 great department of histology, and by that to refuse physiology one of 

 the most important aids ; in fact, an aid which, with the help of better 

 microscopes, in future, is likely to take the lead in the determination of 

 the laws of animal and vegetable life. I am well aware that the study 

 of histology has been pursued with the ordinary instruments, of the 

 German pattern, in a great measure ; but knowing what these have 

 done both in Europe and in this country, and having discovered, by a 

 few glimpses, how much more, and how much better, we might have 

 done, had we possessed one of these highly finished instruments, I can 

 confidently assert, that it is a grave error to tell opticians that they 

 had better devote themselves more particularly to the improvement of 

 the ordinary instruments, and let their transcendental corrections of 

 widely gaping objectives serve in the mean while as playthings for 

 curious amateurs. 



" But it is a still more serious mistake to say to students, that an 

 instrument which performs under a variety of circumstances ' without 

 much sensible deterioration' is practically the best for all ordinary 

 purposes. 



" So thought Ehrenberg, and yet we all now know what curious 

 mistakes he made. Embryology, too, comes under this proscription ; 

 for any one who has attempted to trace the development of animals, 

 especially the lower forms of life, must know that it is impossible to 

 separate the study of their cellular structure from the investigation of 

 their organs. 



" I cannot more fittingly conclude this communication, than by 

 quoting, by Mr. Spencer's leave, a portion of a recent letter of his 

 to me. He says : ' It seems to me that there is every reason to hope 

 much from the earnest application of high powers with large angles. 

 So blind and inveterate has been the prejudice in favor of low powers 

 and small angles, in histology, that younger and less prejudiced micro- 

 scopists have a comparatively untrodden path before them. Every 

 day's thought convinces me more and more deeply of the radical mis- 



