107 



correction of the objectives for perfect vision is not the best for photo- 

 graphic purposes. Accordingly, with all powers lower than the ^-inch, 

 after focussing by the eye on the ground-glass slide, the objective is 

 usually withdrawn a little by means of the fine adjustment. I have 

 found no difficulty of this kind, even when working with the inch 

 objective. 



The ground glass upon which objects are usually focussed, is not 

 delicate enough to show the lines on diatoms, &c. I have, therefore, 

 taken out this glass and substituted for it a piece of plate glass, 

 coated with skimmed milk (just as we coat a glass plate with col- 

 lodion), and allowed to dry. Thus, while taking an ambrotype of the 

 dots on Pleurosigma angulatum, I found that I could not bring out 

 these dots on the common glass slide. But when I substituted the 

 glass, coated with milk, they immediately became visible. 



I used the common ambrotype methed, which is too well known to 

 require any description. By changing the number and proportion of 

 the salts used in the collodion, I obtained a collodion which requires 

 an exposure only of fifteen seconds, when using the -^^ inch objective, 

 and when the sensitive plate is placed at a distance of five feet from 

 the objective. I hope that I may be able to obtain a collodion sensi- 

 tive enough to give an ambrotype of the circulation of the blood in 

 a frog's foot with a ^ or J inch objective. 



My instruments and objectives were made by the Grunows of New 

 Haven. The Jg inch objective, which I have used in obtaining am- 

 brotypes, showing the lines and dots on Pleurosigma angulatum and 

 P. attenuatum, &c., is one of large angular aperture, which belonged 

 formerly to Prof. Bailey, of West Point. 



Amasa M. Eaton. 



Providence, R. I., March 5, 1861. 



Dr. Bryant read the following paper : — 



Remarks on the variations of plumage in Buteo borealis, 

 AucT., AND Buteo Harlani, Aud. ? By Henry Bry- 

 ant, M.D. 



The variations of plumage in the individuals of the species of the 

 genus Buteo, common in the Atlantic States, are so slight that it is 

 not to be wondered at, that the first specimens from other parts of 

 the country, presenting, as they did, such extraordinary variations of 

 color, should have been described as distinct species. At present, 

 however, the number of specimens known is so large, that on careful 

 examination it seems to me necessary to adopt one of two conclusions, 

 namely, either to increase the species indefinitely, or to reduce them 

 to a much smaller number than are at present supposed to exist. As 

 the European buzzard, B. vulgaris, is well known to present the 



