ZOOLOGY AND BOTANY, MICEOSCOPYj ETC, 643 



gives a mueli brigliter field, -while the above-mentioned objection, which 

 in no way diminishes the accuracy of the measurements, has also a 

 certain advantage ; for since povyerful Microscopes are very sensitive in 

 respect of exact focusing, the plane of the scale must be accurately 

 perpendicular to the axis of the Microscope, or the image will not remain 

 clear during a complete rotation ; whereas with the inclined position one 

 part is always in focus. 



Describing the special advantages of his own arrangement of the 

 Microscope which has now been largely used since 1879, the author 

 says : " The great advantage is simplicity ; the few divisions of the 

 micrometer are easily taken in by the eye, so that no other method of 

 measurement is so rapid. Further subdivisions or transverse lines are 

 unnecessary and troublesome, and do not increase the accuracy. A 

 portion of the scale of the instrument is separated by the Microscope 

 into 100 parts ; one-tenth of these are read by the direct divisions of 

 the micrometer, and the tenths of the latter by estimation. The reading 

 is not conducted in any other way except for special purposes. 



If, for example, a circle is divided by one-sixth of a degree, or at 

 intervals of ten minutes, and the micrometer contains ten equal intervals 

 which occupy exactly one division of the circle, each such interval cor- 

 responds to one minute. If the latter can by estimation be subdivided 

 into tenths (by practice even into half-tenths) the unit of reading is six 

 (or three) seconds. Fig. 101 shows the sixth division of a degree on the 

 circle near the ten divisions of the micrometer. The divisions of the 

 circle are numbered from degree to degree with to 9, either by 

 the pantograph or with figures made as small as possible and as near as 

 possible to the lines so that at least one number shall be visible in the 

 Microscope whose field covers more than one degree. It is not then 

 necessary to use a special index or a lens to read the angle; for the 

 principal numbers at each 10 degrees may be made large and placed 

 outside the silver strip where they can be easily seen with the naked 

 eye. If the circle is not covered the illuminator will at once show 

 whether the reading is between 10 and 20 or 30 and 40, &c., and the 

 single degrees are given by the divisions in the Microscope. If the 

 circle is covered it will be necessary to have, in addition to the two 

 small apertures for the Microscopes, a larger one inclosing about 

 15 degrees, at a point 90° from them, and having in the middle of its 

 glass a black line by which the approximate angle is read off. Sup- 

 posing that this line shows the reading to be between 30'^ and 40°, and 

 that the micrometer stands as shown in the figure, the reading will be 

 33°37'-3or33°37' 18". 



In the inverting Microscope the division on the circle always runs 

 towards the long or zero mark of the micrometer, i. e. from left to right 

 when the numbers of the horizontal circle run from right to left. The 

 divisions of the micrometer are reckoned from zero point in the opposite 

 direction, from right to left. With vertical circles where the numbers 

 go from left to right, because the circle turns with the telescope, every- 

 thing is reversed ; in the right-hand Microscope alone the graduations 

 are reckoned from right to left or downwards, and the micrometer 

 divisions upwards; in the left-hand Microscope the graduations are 

 read upwards and the micrometer downwards. For small instruments 

 it is convenient to have the scale divided at intervals of 20 minutes ; a 

 micrometer division is then equivalent to 2 minutes ; in this case it is 



