ZOOLOGY AND BOTANY, MICROSCOPY, ETC. 445 



or fifths. But this becomes difficult if not futile on account of diffrac- 

 tion, imj)erfect definition, inequality in the illumination of the scale and 

 of the object, parallax from tremor in both apparatus and observer, and 

 error in making optical contact between the margin of the object and 

 the line from which measurement is to begin ; elements which bring a 

 large personal equation into the case, as they vary greatly according to 

 the capacity of individual workers and the quality of their outfit. 



The above is intended to show what can be done by a skilful 

 person with good but commonplace apparatus. The ruling may cost 

 perhaps a couple of dollars, and a high-power ocular to carry it, about 

 twice as much. The objective required for the work is not of unusual 

 power or quality ; and any small, jjlain Microscope of fair quality and 

 good fine-adjustment, can be employed, a lengthening tube being 

 improvised if there be no draw-tube. A screw-movement to adjust 

 the lines in the ocular to the image of the object, or else a mechanical 

 stage for adjusting the object to them, will be of great assistance ; but 

 as the latter, of efficient character and applicable to the most unpre- 

 tending stands, can now be made for 18 dollars, it is not a very un- 

 reasonable luxury." 



Glass versus Metal Micrometers.*^ — Prof. M. D. Ewell writes : — 

 " I think most persons who use stage micrometers in the ordinary way, 

 prefer to have them covered, on account of there being less danger of 

 injury and their always being ready for use. When my experience was 

 less than it is now, I remember attempting to clean a really excellent 

 micrometer by Prof. Eogers, 1 cm. long, ruled the whole length to 

 0"001 mm. I found out that it was uncovered after I had scoured the 

 lines vigorously. It was then clean, but that was its only remaining 

 recommendation. 



Prof. Eogers has experimented much to avoid the sweating that 

 so often obscures the lines when the cover-glass is secured in place by 

 any kind of cement. The most successful method, I think, has been to 

 rule the scale on a cover-glass and mount it with the lines downward, 

 upon a thick ring perforated, so as to allow a free circulation of air. 

 This, again, has its peculiar disadvantages, as I have learned after the 

 point of my objective (a 1/25 Spencer) had gone through the cover. 

 The lesson was more impressive after I had paid Mr. Spencer's bill for 

 re-centering the front lens. Micrometers so mounted are very fragile 

 unless the cover-glass is too thick for ordinary use. In a later commu- 

 nication I shall describe a device of my own to prevent the sweating 

 above alluded to. 



Another disadvantage of micrometers ruled on glass is the fact that 

 there is always more or less uncertainty as to their staying qualities for 

 some time after they have been ruled. This, so far as I have observed, 

 is peculiar to all lines ruled on glass ; for I have observed them not 

 only in scales ruled by myself, but on those by Prof. Eogers and Mr. 

 Fasoldt. I do not say that this is universal , but it happens often enough 

 to make the possessor sad. The makers are not be blamed for this ; for 

 it seems due to an infirmity of the material. The only remedy is to let 

 scales on glass season for an indefinite time, like thermometers, before 

 issuing them. 



My own judgment is that the very best scales are ruled upon metal. 



* The Microscope, ix. (1889) pp. 43-5. 



