ZOOLOGY AND BOTANY^ MICEOSOOPYj ETC. 571 



the balsam being at the same time spread out and made drier and 

 stiffer. The field-lens is now screwed in, and the eye-piece held to 

 the light and examined, to see if the thread is straight ; and if it be 

 found to have in it a number of small flexures due to the weaving, it 

 is best to replace it by another. So far there is no particular difficulty, 

 but the next step, which is to attach another fibre parallel with the 

 first and very close to it, is rather a delicate operation, as it must be 

 done without disturbing the first, and the distance apart of the two 

 must be regulated with the utmost nicety. This distance will of 

 course depend on the space to be measured; assuming that it is 

 desired to measure 1-lOOOth in. with a C eye-piece and a l-4th in. 

 objective, it will be about l-30th in. The field-lens having been 

 replaced, the eye-piece is inserted in its proper position, and a stage- 

 micrometer laid on the stage, with the thousandth divisions in the 

 field (under the l-4th in.) ; the eye-piece being now placed so that the 

 silk lines are parallel with the ruled lines on the stage-micro- 

 meter, it can at once be seen whether the fibres are too close or too 

 distant. If either is the case the field-lens must be removed and one 

 of the fibres shifted, which may be done without much difficulty with 

 the blunt-pointed penknife, pressing down the ends in the balsam 

 and at the same time gently rolling them almost imperceptibly to one 

 side. The field-lens is again replaced and the eye-piece placed in 

 position, and examined, and if necessary the operation repeated till 

 the two fibres are exactly coincident with two of the lines on the 

 stage-micrometer, when the space between them, as seen on the 

 magnified image, will represent 1-lOOOth in. I have fixed a third 

 line at the same distance as the second from the central one, or 

 1-lOOth in. from each other. These last two threads are near the 

 opposite sides of the diaphragm, their actual distance apart being 

 about 7-16ths in. 



Finding the balsam scarcely sufficient to keep the fibres tight I 

 cut very small narrow slips of postage-stamp paper, moistened them, 

 and fastened down the ends of the fibres with them, at the same time 

 pulling them " taut," with a slight pressure, but taking care not to 

 move them laterally ; there is a good deal of risk, however, of the 

 latter, and as the mucilage dries immediately, it will be necessary, 

 should the fibres prove on examination to be displaced, to remove 

 the paper forcibly, most likely breaking the fibre, and necessitating 

 its renewal. The dots of balsam being placed well back from the 

 margin of the diaphragm, there is room for the attachment of the 

 slips of paper between them and the edge. If a transverse line is 

 desired crossing the others, it is best to fix two fragments of the 

 paper on the opposite sides of the diaphragm, and mount the ends of 

 the thread upon them (fastening them down with other pieces), so 

 that the fibre is clear of the others, and there is no risk of dragging 

 them out of place in attaching it. 



The eye-piece, as described, gives direct measurements of 1, 2, 

 4, 5, 6, and 10 thousandths of an inch. The lines on an ordinary stage- 

 micrometer appear with the l-4th in. fully six or eight times as thick 



