1883.] 



MICROSCOPICAL JOURNAL. 



179 



still 1823 somewhere out in the Central 

 Pacific when the eclipse occurred, so 

 there 1823 could be credited with seven 

 eclipses. 



CORRESPONDENCE. 



Divisions of jMicrometer Eye-piece. 



To THE Editor : — Will you kindly 

 tell me what is the best way — in what 

 parts of an inch — to have an eye-piece 

 micrometer ruled ? I shall want to use 

 it not only to measure objects microscopi- 

 cally large, but objects that are at the 

 other extreme and are microscopically 

 minute. 1 find nothing in the books in my 

 possession to help, except your own arti- 

 cle " How to measure," in the American 

 Monthly Microscopical Journal. 

 You there say the lines may be about 

 yigth inch apart, isn't that space pretty 

 large for minute objects ? 



[The above letter was not sent for pub- 

 lication, but the question presented is of 

 such general interest that we have de- 

 cided to answer it in the Journal for the 

 benefit of other subscribers as well as the 

 writer. 



In the first place, the reader must un- 

 derstand that the exact distance between 

 the lines is a matter of no consecjuence. 

 When we speak of y^-jj of an inch as a 

 convenient spacing, it is not necessary 

 that the lines should be precisely yoU ^^ 

 an inch apart. It is only necessary that 

 thev be perfectly spaced, whether the 

 unit be yJo? y-io' o^ oV o^ ^""^ inch. The 

 value of the divisions is given in terms 

 of a standard stage-micrometer, and has 

 no reference whatever to the proportion 

 that may exist between the spaces and 

 an inch or centimetre on a common 

 rule. To illustrate this, suppose we 

 place a ruled eye-piece micrometer in 

 place and put a stage-micrometer ruled 

 in hundredths of an inch on the stage, 

 using |-inch objective. The image of 

 the stage-micrometer is seen in conjunc- 

 tion with the lines in the ocular. As the 

 draw-tube is moved out or in the image 

 of the stage-micrometer grows larger or 

 smaller, while the distance between the 

 eye-piece lines remains unchanged. In 

 one position each division in the ocular 

 may equal eight of the stage-micrometer, 

 in another position, nine. In the one 

 case it represents j|g of an inch, in the 

 other y^. It has, therefore, no constant 



value except for a certain objective with a 

 stated length of draw-tube. 



As regards the best spacing for an eye- 

 piece micrometer, it is clear that the 

 value of the divisions changes with the 

 objectives. As the power of the latter 

 is increased, the value of each space in 

 the eye-piece scale becomes less, and 

 when very low-powers are used the value 

 of the divisions increases accordingly. 

 Consequently, it will be found, in prac- 

 tice, that the best division for one power 

 will answer very well for any other 

 power. We are referring now to eye-piece 

 micrometers of the ordinary kind — plain 

 lines ruled on glass, every fifth and tenth 

 line longer than the others. It might be 

 assumed that very close lines would be 

 desirable to accurately measure extremely 

 minute objects, and much more widely 

 spaced ones for larger objects. But in 

 practice it will be found that when lines 

 are quite close together in the ocular 

 they are confusing to count, and they ob- 

 scure the view, so that more accurate 

 measurements may be made with lines 

 more widely separated, leaving the more 

 minute division to be estimated. On the 

 other hand, when the hues are too far 

 apart the fractions of the divisions can- 

 not be so accurately estimated. On the 

 whole, therefore, it seems best to adopt 

 some division which shall give sufficient 

 accuracy without confusing the mind in 

 counting the lines, or in any wise obscur- 

 ing the view of the object. It is on these 

 grounds that we have ventured to recom- 

 mend the spacing of y^-q of an inch. 



It should be remembered that the 

 eye-lens of the ocular magnifies the eye- 

 piece scale, and the apparent distance 

 lietween the lines will vary with the 

 l)0wer of the eye-lenses of different 

 oculars. It is the apparent size of the 

 scale that concerns us in this matter, 

 more than the actual spacing. We pre- 

 fer to use the scale in the " B " ocular, 

 making it a permanent fixture in the 

 ocular. — Ed.] 



Diatoms In Barbadoes Deposit. 



To THE Editor. — Some time in June 

 I received from a correspondent a small 

 vial of a new Find of Barbados polycys- 

 tina, which when I came to mount, I 

 found to contain what to me was a new 

 form of diatom, and I mounted three 

 valves and sent them to Mr. Chas. 

 Stodder who answers, as follows : The 

 slide No. 3 is a curious mutation-varia- 

 tion of the coscinodisctis J never saw the 



