WITH THE MICROSCOPE. 41 



in the sum, divide by 4, and shift the decimal point one place to the 

 left, thus to 9 t- 84 add t- i5 and the sum 6-999 "^ 4 == 2 '498, and 

 shifting the decimal point mm- 2498 which does not differ sensibly 

 from mm -2 5, the correct quantity. 



A French line contains 'oSSS English inches. To convert lines 

 into thousandths of an inch, shift the decimal point one place to the 

 right, and multiply by 9 ; if greater accuracy be required, subtract 

 ii from the second place of decimals for each of the nearest number 

 of units in the product. Thus o'" - i25 becomes 1^25 which x 9 = 

 n t- 25, from which subtract t- i4, and the value in thousandths is 

 found to be ii t- io, which is correct. 



To count thousandths into lines add i~ in the second place of 

 decimals for each of the nearest number of units in the sum, divide 

 by 9, and shift the decimal point one place to the left, thus to 

 i J'-TO, add t- i4, the sum ii'25 divided by 9, and the decimal point 

 shifted one place to the left gives o'" - i25 as before, 



In most cases it will be unnecessary to apply the corrections 

 noticed above, but by remembering the short rules given, any one on 

 reading a foreign work may correct the measurements as he reads, 

 and insert them in the margin without delay or interfering with his 

 progress. 



Method of finding the same Spot in a Specimen. 



67. Of marking the Position of an Object. Various plans have 

 been proposed from time to time for marking the exact position of a 

 minute object in a specimen, so that it can be placed in the field of 

 the microscope whenever required. A fine line of varnish or Bruns- 

 wick black may be drawn round it, or a small and very thin metal 

 tube (about the tenth of an inch in diameter) may be moistened with 

 the varnish and pressed upon the glass cover, so as to encircle the 

 particular object required with the line. 



Mr. Bridgman, of Norwich, has designed an instrument for drawing 

 a circle upon the thin glass with a diamond point (Microscopical 

 Journal, vol. Ill, p. 237). This instrument is represented in pi. XIII, 

 fig. 67. A is a brass cap fitting upon the end of the object-glass, 

 which it entirely covers up and protects from injury ; B, a stem 

 soldered to the side of the cap with the upper end having two pro- 

 jecting sides to steady the ends of C, e, and f, which are firmly 

 secured to it ; C, an elastic arm of hammered brass, which carries 

 at its lower end D, a lever of thin brass plate, having a fragment of 

 diamond inserted in its thinner end, and directly under the centre 

 of the cap A; e and /are two springs, pressing upon the shorter end 



