File:The microscope and its revelations (1901) (14580418188).jpg

Identifier: microscopeitsrev00carp (find matches)
Title: The microscope and its revelations
Year: 1901 (1900s)
Authors: Carpenter, William Benjamin, 1813-1885 Dallinger, W. H. (William Henry), 1842-1909
Subjects: Microscopy Microscopes Natural history
Publisher: Philadelphia, P. Blackiston's Sons and Co.
Contributing Library: MBLWHOI Library
Digitizing Sponsor: MBLWHOI Library

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and scientificmethod. It has been advised that a record of comparisons with thevarious lenses in the possession of the microscopist should be madeonce for all. We decidedly deprecate this method, unless it be insuch utterly valueless work, as is sometimes done, where lenses are(incorrected and accuracy of tube-length forgotten or ignored. Thecorrection of an objective and the tube-length ought to vary withevery object, and therefore a comparison of the stage-micrometerand the screw-micrometer should lie made with every set of measure-ments. Moreover, the majority of -ta^e micrometers exhibit very con-siderable discrepancies in the several intervals between the line*:it is \\ell in the interests of ace u racy to take t he screw value of eachunder a high power, liud the value of the average, and then note theparticular space or spaces that may lie in agreement with the averageand always use it. An illustration will make this clear. provides a stage micrometer of I mm. divided into 1 and
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en a 2; 5<H r :; - - X ao w a TO OBTAIN THE VALUE OF A MICROMETER INTERVAL 275 •01. The following are the actual values obtained for each of the 05 divisions, viz. :— o4U 8-378-388-388-368-368-588-338-318-478-338-338-388-448-388-408-378-408-2-,8-38 838 mean value. In this instance it will lie seen that the last division. 838, agreeswith the mean, and is the best for all future use.1 Having thus obtained a screw-micrometer value for a certainknown interval, the screw-micrometer value for any other objivtbeing known, the size of the object may be found by simple propor-tion ; thus. viz. if 838 is the screw-micrometer value for 05 mm.and 6-45 that for a certain object, the size of the object is (i) 8-38 : 6-45 :: -05 : a; mm.; 6-45 x -05 n9QK- = - =0o85 mm. 838 If the answer is required in fractions of an English inch, all thatwe need remember is that 1 inch=25-4 mm.; then .;i-,(ii) 8-38 : 645::o_° : ..: inch ; 6-45 x-00197 -0127.<•= -= =-001515 inch. 8-38 s-38 If the stage-

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