OF ARTS AND SCIENCES. 141 



section-like precision of the flat field, wliicli at once revealed to the eye 

 the exact and relative level of every vesicle or yolk-cell. 



" I "was most forcibly reminded, not long ago, of the value of the 

 wide angle of aperture, and the accompanying great amount of light, 

 upon trying Spencer's objective upon the stem of a well-known Hy- 

 droid, the Clava leptostyla, Ag. In the manuscript of the forthcoming 

 volume of Professor Agassiz's " Contributions to the Natural History of 

 the United States of America," the outer wall of this Hydroid, and of 

 several others, I may say in passing, had been described as a structure- 

 less membrane ; but what Avas my surprise, in my last attempt, to find 

 that this wall was composed of a layer of polygonal cells, as distinct as 

 any in the other parts of the animal, and even readily discernible in 

 the more opaque parts, where the stem appeared like a simple black 

 surface under the ordinary microscope. 



" In regard to the usually estimated Avorth of wide angles of aper- 

 ture, I would say, that, from numerous experiments upon living tissues, 

 objectives having this property are valuable, not so much because they 

 can admit extremely oblique one-sided rays, but because they allow rays 

 to enter from all sides at a very wide angle to the axis. One-sided 

 oblique rays throw the shadow, in a great measure, beyond any par- 

 ticular cell upon its neighbor, and this produces distortion ; Avhereas 

 when the rays converge at a wide angle, each cell becomes strongly 

 marked at its periphery by a dark, broad shade. A moderately 

 oblique, one-sided light, hardly tAventy degrees from the axis of the ob- 

 jective, ahvays appeared to be the most frequently serviceable. I was 

 surprised one day to find that the hitherto faintly visible circulation in 

 the cells of Spirogyra was rendered, by such a light, very distinct, and 

 the granules borne along in the current appeared like little specks with 

 a very sharp, thick, black outline. 



" At first thought, there would appear to be an insuperable objection 

 to the Avide angle of such objectives, and that is the shortness of the 

 Avorking distance, Avhich Avill not alloAV one to take anything more than 

 a superficial vieAv of a body, even of moderate thickness. But this 

 objection has not the least force, and, on the contrary, the more 

 nearly absolutely flat the field is, especially in the lower powers, such 

 as the ^, f , and 1 inch, the better will they bear the use of the 

 higher eye-pieces. This is not a speculative suggestion, for I have 

 been told by Mr. Spencer, that he has been able to see the lines upon 



