Micrometry. 237 



of these, in our opinion, that should b^determined by our 

 standard micrometer. But, as we have defined the term unit, 

 either of these values would be too large for convenience. The 

 i-iooo mm., the so-called micro-millimeter, or micra, of the 

 French (designated yu), appears to us more suitable for our 

 unit. A few examples of its application may serve to support 

 this view. 



The diameter of a human blood-corpuscle is about 7.7 fj. 

 (.0077 mm.). In birds, the corpuscles measure from 12 to 14 // 

 in one direction, and from 6 to 8 /< in the other (.012-.014 mm. 

 X. 006-008 mm.). The corpuscles of Proteus anguinus measure 

 58 //, and still larger are those of Amphiiima tridactylum 175 yw 

 (.058 and .175 mm.). 



If the reader will assume some other unit, as the cm., or 

 mm., or even the i-ioo mm., and endeavor to express the same 

 dimensions in these terms, the advantage of the micra will, we 

 think, be obvious. We would prefer this unit to the hun- 

 dredth of a mm., already proposed, and urge its considera- 

 tion by the American Society of Microscopists. Thus, although 

 much has been said against the i-ioo mm. unit as being too 

 small, and although the New York Society has deemed it 

 necessary to reconsider and retract the resolutions passed at 

 first commending the action of the Congress in adopting it, 

 we are decidedly of the opinion that the still smaller unit of 

 i-iooo mm. is the more desirable, and it has the prestige of 

 actual use in France, and is generally known. 



The reader will observe the distinction we have made above 

 between the standard division of the micrometer, and the unit. 



The former constitutes the accurate scale of reference ; the 

 latter a subdivision of that scale, obtained by an eyepiece 

 micrometer. The distinction is made because the most con- 

 venient rulings on a stage micrometer are the i-ioo mm., while 

 the adoption of the same divisions as a unit would involve the 

 use of longer decimals. With the micra, a single decimal is 

 sufficient for ordinary purposes. 



A SHOWER OF POLLEN IN PENNSYLVANIA. 



We are indebted to two correspondents for specimens of the 

 pollen which is described in the following abstract from Mr. 



