768 SUMMARY OF CURRENT RESEARCHES RELATING TO 



tions and correction of our errors of vision, our mistaken or imperfect 

 views, by mutual interchange. 



2. The use of the Camera Lucida. — A second means of verification 

 is the use of this instrument, by which we may record in permanent 

 form the fleeting vision of a single observation. Our memory is im- 

 perfect even when strongly impressed ; we forget the details of what 

 w^e see, or confuse them with later images. But a drawing preserves 

 the forms we have observed secure against memory's obliteration, and 

 at the same time, by the very act of drawing, our attention is quickened 

 and our recollection made more clear. A drawing thus made serves, 

 too, a double purpose. It preserves for us a transcript of what we saw 

 to compare with our own later studies, and it serves as a ready means 

 of interchange of views with others, outweighing, often, many a page 

 of mere description. The various new and improved forms of the 

 camera lucida which have been brought out in the past year or two 

 are therefore matters of congratulation. 



3. The use of Photomicrography. — This is, perhaps, a still more 

 important adjunct for verification. By this beautiful application 

 of the art preservative we have not alone a quick and easy mode of 

 obtaining a record of our observations, for comparison with those of 

 others, or with our own later studies, but we have a record that is 

 almost entirely free from the fallibility in^.erent in a mere drawing. 

 Photography has errors of its own, but it eliminates the errors of the 

 hand and eye and judgment. The shadowy distortions projected 

 into the microscopic image by our imagination disappear when the 

 light writes down its own impressions of the structure it traverses. 

 And thus the photographic evidence of what can be seen is a verifica- 

 tion indeed. The service it wrought in the hands of Dr. J. J. Wood- 

 ward in first demonstrating the resolution of the finer diatoms and of 

 Nobert's higher bands of ruled lines by American objectives, you all 

 remember. The photograph itself is not free from possible error ; it 

 cannot focus itself, it will record diffraction images as well as 

 negative or dioptric ones, and hence its own record needs careful 

 interpretation. As in astronomical work the photographs of the 

 comet or nebula, of the eclipse of the sun, or the transit of Venus, do 

 not give the final truths that are sought after, but need to be carefully 

 collated and measured and studied in many ways, that from them may 

 be deduced the structure of the corona or the comet, the parallax and 

 distance of the planet and the sun — so with the photomicrographs of 

 the objects of our study. They may not be absolute proofs on their 

 face of the real structure under examination, especially in the case of 

 very minute lines or particles near the limit of visibility, but they 

 present a record of that structure, freed from the ' personal equation ' 

 of the observer, and they preserve that record for study and com- 

 parison in the indefinite future, when details now unthought of, and 

 therefore unnoticed by the eye, shall be seen to be of importance in 

 its interpretation. And may not the photograph do even more than 

 this in microscopic verification ? Its achievements in astronomy and 

 in recording the swift motions of the racehorse may yet be duplicated 



