MEMORANDA. 241 



nor in Rabenhorst, Nor did any English observer know it. 

 But I now find that Ehrenberg had described it as P. borealis^ 

 ten or twelve years ago, although his figure, which, if pub- 

 lished, appeared in the Berlin Transactions or the Berlin 

 Monthly Reports, was entirely unknown to all our authorities 

 in this country, none of whom, more than myself, have been 

 able to consult Ehrenberg's very numerous papers on the 

 Berlin Transactions or Monthly Reports, except as quoted by 

 Kutzing or Rabenhorst, neither of whom noticed this species. 

 I mention these facts to explain how it was tiiat a species 

 long ago described, and 1 believe figured, by Ehrenberg, was 

 regarded by all our authorities as new when I found it in the 

 Mull earth two years ago." 



Allow me briefly and respectfully to state, that the Pin- 

 nularia horealis is figured by Kutzing, and is figured by 

 Rabenhorst ; and that Kutzing copied his figure from Ehren- 

 berg's American Tabulae, a work by no means unknown in 

 this country, — J. 



On Micrometers and micrometry-. — In the last number of the 



' Microscopical Journal,' there is a paper b}' Dr. Robertson, 

 quoted from the ' Monthly Journal of Medical Science,' re- 

 commending an ingenious form of eye-piece micrometer, pro- 

 posed by Herman Welcher, a medical student at Giessen, on 

 which I wish to offer a few observations. 



Micrometry, as affected by the compound microscope, 

 consists in the comparison of the magnified image of the 

 object with the similarly-magnified image of a body whose 

 dimensions are known, the most convenient for the purpose 

 being a piece of glass ruled with fine divisions, called a 

 starve micrometer. This comparison cannot, as Dr. Robertson 

 correctly observes, be made directly by laying the object on 

 the divided scale ; but it may be made indirectly., either by 

 the camera lucida, as practised by Mr. Lister, or by means of 

 an eye-piece micrometer. The latter method, in addition to 

 convenience in application, has the further advantage of sub- 

 dividing the divisions on the stage to an extent corresponding 

 to the magnifying power employed ; but it also has the dis- 

 advantage of enlarging their errors in the same proportion. 



In ruling a micrometer, the elasticity of the materials of the 

 dividing engine, the friction of its moving parts, or the free- 

 dom of motion necessarily allowed in order to diminish that 

 friction, will produce a very slight inequality in the individual 

 divisions ; but if these be carefully examined, their errors 

 will generally be found to be alternately plus and minus, in no 

 case cumulative. It therefore follows that the sum of the 

 errors of a number of divisions will scarcely ever exceed that 



