Baled 

 micrometer. 



Micrometer 

 eyepiece. 



68 THE MICROSCOPE 



ttousandths of an incli has been ascertained with a particular 

 object glass and a particular tube length, the measurement can 

 be made direct in the microscope. For this pur- 

 pose ruled squares in the eyepiece are not always 

 convenient. An eyepiece micrometer is a plate of 

 glass ruled with a finer series of lines (Fig. 73). It 

 drops into the eyepiece in a similar maimer to the 

 square ruling. An even better method of making 

 such measurements is by means of the Beck mi- 

 crometer eyepiece. 



This consists of a complete eyepiece with a 

 magnifying power X 8, and a special vernier milli- 

 metre scale (A, Fig. 75) placed in its focus which is outside the 



Fia. 73. — 

 No. 3276, 

 Eyepiece 

 Micrometer. 



lit is provided with a coUar (B) which fits over the draw- 

 tube and can be clamped in position by a milled head (C). The 

 eyepiece itself can be focussed up and down by revolving it in 

 its fitting till the scale A is in exact focus for the observer's eye. 



The scale (Fig. 76) is in millimetres with a vernier reading to 

 1/lOth of a mm. 



On the left is a vertical series of divisions divided in haH- 

 milUmetres for rough measurement. For fine measurement the 

 object to be measured is placed in a horizontal position, and the 

 length is measured in 1/lOth mm. by use of the slanting line on 

 the right. The image of an object as shown in the diagram 

 measures 3*25 mm., because it covers three large divisions 

 and extends to the oblique line at a point half-way between the 

 •2 and |'3 of tenth-nuUimetre vernier divisions. 



To obtain the actual size of the object itself, this result has 



-B 

 —A 



4 J z I a 



Fig. 74. — No. 3275, 

 Micrometer Eyepiece . 



Fio. 75. —No. 3275, 

 Micrometer Eyepiece. 



Fig. 76.— Scale 



of Micrometer 



Eyepiece. 



merely to be divided by the initial magnifying power of the object 

 glass. (See table of magnifying power on page 77.) 



In cases where great accuracy is required, each object glass 

 can be verified as to its initial magnifying power by the use of a 



