274 ANNUAL RECORD OF SCIENCE AND INDUSTRY. 



would so far coincide with the true lines as not to prevent 

 satisfactory definition ; and he suggests, for the purpose of 

 testing the theory of Helmholtz, the study of fine lines at 

 very close but unequal intervals, with one or two missed out 

 here and there. Theory indicates that such tests would be 

 far more difficult to see correctly than lines ruled at regular 

 and equal intervals. Monthly Microscox>. Journ.^July^ 1876. 



BLOOD-CORPUSCLES OF THE BATEACHIA. 



Professor Gulliver states that the blood -globules of the 

 Batrachia are generally flat, or somewhat vaulted ; in out- 

 line a well-defined oval, mixed occasionally with circular 

 forms. Those of the Amphiuma are visible to the naked 

 eye, and those of Proteus next in size the latter thrice as 

 laro;e as in frosfs and toads; an amount ofdifi(erence of which 

 there is no example either in the class of birds or reptiles. 

 Proceedings Zool. Society^ February, 1876. 



HELMHOLTZ ON LIMITS OF VISION. 



In this paper it is stated that diflTraction of the rays is 

 beyond doubt the principal cause of the limitation of sharp- 

 ness of the microscopical image. In comparison with diffrac- 

 tion, chromatic and spherical aberration appear to exert but 

 an inconsiderable influence, in spite of the very large angles 

 of incidence and divergence of rays. Considering the ex- 

 treme care expended on calculation and execution of lenses 

 for telescopes and the photographic camera, it is justly a 

 matter of surprise that with the lenses of the microscope, 

 which are so much more difficult to construct according to 

 the prescribed dimensions, and which have so large an aper- 

 ture, spherical aberration makes itself so little felt. We may 

 add that while undoubtedly theory has very largely contrib- 

 uted to the perfection of the lenses for telescopes and came- 

 ras, it has hitherto done little, and, indeed, from the very 

 nature of the case, can do comparatively little, for the perfec- 

 tion of the microscopical objectives. Almost all the makers 

 of such lenses we might say all of any note depend upon 

 acquired skill in the use of certain tests, i. e., the artificial 

 star, as indicating the necessary changes, in laboring toward 

 perfection ; and very seldom, we venture to say, has such 

 perfection been the result of a rigid adherence to curves. 



