702 



RECOED OF CURRENT RESEARCHES RELATING TO 



individualities of the workers' eyes, of wliich one of the most important, 

 but least thought of, is astigmatism. Owing to this defect, the^ later 

 pictures of Turner are found to be distorted, the tendency being to 

 exaggerate the size of the paler dimension in painting it. On the 

 contrary, in microscopical drawing (as with the camera lucida) the 

 improperly pale line will be perpetuated and the perspective mis- 

 represented. Distortion of dimensions generally may be perpetrated 

 by the most careful observers, and endless disputes may thus arise. 

 A familiar example of this is shown in the case of the Podura scale. 



Micrometre or Micromillimetre.* — Dr. Phin points out that 

 " micrometre " is inadmissible in America at least, as it would there 

 be spelt " micrometer," and confounded with the instrument of that 

 name, a difficulty which the difference in pronunciation would not 

 remedy. He thinks, therefore, that the proper way is to " fall into 

 line " with the British Association Committee, and adopt the nomen- 

 clature suggested by Mr. Stoney, calling the thousandth of a milli- 

 metre (or the millionth of a metre) a — -, or sixth-metre — the prefix 



sixth here indicating the negative exponent of 10 by which the metre 

 is to be multiplied. 



Micrometry and CoUar-adjustment.t — Dr. Beale, in his ' How to 

 Work with the Microscope,' recommends that scales be drawn or 

 printed, showing the size to which hundredths or thousandths of the 

 inch or centimetre are magnified by each of the objectives used, and 

 one of these scales corresponding to the objective employed, pasted on 

 every drawing. A writer in the ' American Monthly Microscopical 

 Journal ' recalls the fact that in all objectives made with a collar- 

 adjustment, the magnification at the " open " and " closed " points 

 varies so much, that attention to this is necessary in making the scales 

 as suggested. Whilst the fact is well known, the amount of the 

 difference does not seem to have been sufficiently taken into account. 

 This will be best illustrated by a table showing the variations in a 

 few objectives of well-known makers, taken with a tube 10 inches in 

 length, measured from the stage-micrometer to the end of the tnbe 

 proper (not to the end of the eye-piece) : — 



' Am. Jon in. Micr.,' p. 117. 

 Am M. Micr. .loiim.,' i. (1880) p. 67. 



