1881.] 



MICROSCOPICAL JOURNAL. 



191 



How to Measure Objects. 



So many microscopists find diffi- 

 culty in making measurements with 

 the microscope, that we have deemed 

 it worth while to describe the sim- 

 plest process with some care. 



An eye-piece micrometer is invalu- 

 able for work of this kind, and the 

 simplest form is the best. A disc of 

 thin glass with a series of lines ruled 

 upon it, every fifth and tenth line 

 being longer than the others, placed 

 within an ocular in the focus of the 

 eye-lens, forms the most convenient 

 and cheapest arrangement, and one 

 sufficiently accurate for ordinary pur- 

 poses. The thin glass does not ma- 

 terially affect the definition, and it 

 may remain in place ready for 

 instant use. It is advisable to place 

 it in the " B " eye-piece. The lines 

 may be ruled about one one-hun- 

 dredth of an inch apart, and they 

 should be short and distinct. Meas- 

 urements should be made in the cen- 

 tre of the Held. When the value of 

 the divisions has been determined 

 for each objective, the figures may 

 be arranged for ready reference, as 

 shown in the following table, which 

 is a copy of one we have had in con- 

 stant use for a long time. 



With double nose-piece attached. 



8 



10.0 

 2.9 





.00044 

 .000115 



The first column gives the focal 

 length of the objective. In case of 

 higher powers it would be necessary 

 to give the figures for the collar- 

 adjustment in several positions, but 

 for an ordinary fifth the definition is 



good enough for measuring without 

 changing the adjustment. 



The second column gives the posi- 

 tion of the draw-tube, which is gradu- 

 ated in centimetres. Practically, it 

 is not worth while to give the figures 

 for the different positions of the 

 draw-tube, as it is advisable to always 

 use the latter in one position. To 

 use this table we bring the object to 

 be measured into the middle of the 

 field, and count the number of divi- 

 sions in the eye-piece micrometer 

 that exactly cover it. Suppose we 

 use a ^-inch objective, and find that 

 a filament of an alga measures just 

 4j4 divisions. Then the true diame- 

 ter is 13.3 X 4-5 = 59-85^ or ^h 

 = rJ-^ of an inch, or .00052 X 4.5 = 

 ,00234 of an inch. 



To determine the value of the di- 

 visions of the eye-piece micrometer 

 in terms of the stage-micrometer, fo- 

 cus the latter by screwing the eye- 

 lens out or in until the lines are 

 clearly defined. Then focus the lines 

 of the stage-micrometer, and deter- 

 mine the number of divisions of the 

 former that are equal to one division 

 of the latter. Suppose the stage- 

 micrometer is ruled in thousandths 

 of an inch, and that nine divisions in 

 the eye-piece are equal to two divi- 

 sions on the stage, or two one-thous- 

 andths of an inch. Then 9-^2 = 

 4)4 = .001 of an inch, and one divi- 

 sion = .001 -^ 4.5 = .00022 which, 

 in vulgar fractions, is -nrloTTTT = t^Vt- 



To obtain the value in micras 

 (thousandths of a millimetre, written 

 thus : 0.000,001), it is desirable to 

 use a stage micrometer with lines 

 giving hundredths of a mm. instead 

 of thousandths of an inch. But with- 

 out such a micrometer it is a simple 

 matter to make the calculation from 

 the results obtained as above. Thus, 

 one division was found to represent 

 .00022 of an inch. One thousandth 

 of an inch is equivalent to 25.34/^ 

 (more accurately 25.399772), hence i 

 division = .2222 x 25.34 = S-57^' 



We have been thus particular in 

 describing this simple process, be- 



