6o 



NATURE 



[March 20. 19 13 



divisions of the eyepiece micrometer indicated by 

 the steps (see Fig. 1). Each of the smaller divi- 

 sions therefore represents a micron. 



II exact coincidence between the eyepiece and 

 stage scales does not occur with the proper tube- 

 length, it should be varied — a slight variation is 

 all that is necessary — and the new tube-length 

 recorded for micrometer purposes. 



It is obvious that as a 4 mm. (£) objective has 

 half the magnification of one with a focus of 

 2 mm., such an objective treated the same way 

 will give us the ten divisions of the stage micro- 

 meter covering five of the large divisions of the 

 evepiece micrometer; hence to obtain microns 

 we must multiply by 2, and this is all the arith- 

 metic needed. 



It also follows that with an 8 mm. ($) we must 

 multiply by 4, and with a 16 mm. (§) by 8, to 

 obtain the number of microns subtended by each 

 of the smallest divisions of the eyepiece micro- 

 meter. 



It will be seen then that one of the results of 

 the new departure is to obtain for each objective 

 and for a given tube-length convenient, and in the 

 majority of cases integral, micrometer values, 

 which greatly facilitate the use of the instrument. 

 The actual tube-length differs in most cases but 

 little from the standard length. 



Dr. Metz in his paper gives the value of the 



ter scale showing steps. 



unit of the scale and the proper tube-length to be 

 used with each of the twenty-four of the 

 achromatic, fluorite and apochromatic objec- 

 tives produced by the Leitz firm. 



But, of course, the new micrometer can be used 

 with any objective, and for general purposes it 

 will be employed with objectives having foci of 

 2, 4, 8, or 16 mm. focus. These we have 

 already considered. 



The following table gives the tube-length results 

 obtained in a trial of the new micrometer with 

 objectives of different makers ; it will also show 

 the wonderful simplicity brought about : — 



To demonstrate the simplicity of the method and 

 the degree of accuracy to which the step micro- 

 NO. 2264, VOL. 91] 



meter lends itself, the following examples may be 

 given : — ■ 



The object selected was a valve of Surirella 

 gemma; its length was measured first by an eye- 

 piece micrometer of the usual type and then by the 

 step micrometer. 



(1) Leitz objective J in., possessing micrometer 

 value C00349 mm., length of valve 30^9 intervals 

 of the scale; therefore 30"g x 0^00349 = o" 1078 mm. 

 = io7"8 /i. 



With the step micrometer the value of the same 

 objective is 2 /x at a mechanical tube-length of 

 178 mm., the valve covers 53"8 intervals of the 

 scale ; therefore 538 x 2 = 1076 /x. 



(2) Leitz objective y 1 .. in. oil immersion, micro- 

 meter value = 000164, length of valve 65'5 inter- 

 vals of the scale; therefore 65"5 x 0^00164 = 0^1074 

 mm. = 1074 /1. 



With the step micrometer the same objective 

 possesses a micrometer value of 1 jx at a mechanical 

 tube-length of 168 mm., the valve covers 107" 5 

 intervals of the scale; therefore 1075 x t = io7'5 11. 

 In certain cases of frequent occurrence the use of 

 the eyepiece micrometer involves difficulties. The 

 usual eyepiece micrometer has very fine lines, and 

 with some objects it is difficult to see them under 

 unfavourable conditions of lighting. During pro- 

 longed observations with an eyepiece micrometer 

 this is very fatiguing and apt to strain the eye. 



This defect is particularly pro- 

 = nounced when an object and a micro- 



meter scale are seen by dark-ground 

 illumination, a method which is now 

 largely employed. Indeed, in a 

 g dark-ground field the micrometer 



scale may refuse to come into 

 view. 

 In the new micrometer the intervals are ar- 

 ranged in groups or steps of ten, each group 

 being indicated in an unmistakable manner by a 

 black echelon rising from the first to the tenth 

 interval. This arrangement possesses the great 

 advantage that the divisions can always be seen 

 distinctly whether the objects be light or com- 

 paratively dark. 



The micrometer is mounted on the diaphragm 

 of the eyepiece, and can be sharplv focussed with 

 the eye-lens, which is mounted in a sliding sleeve. 

 The device is made by E. Leitz, and its cost with 

 eyepiece is fifteen shillings. 



Double Demonstrating Eyepiece. 



Next in importance to the new micrometer 

 comes a form of eyepiece, introduced also by the 

 firm of Leitz, which enables two observers to use 

 the same objective, and therefore to view the 

 same object. It is called a double demonstrating 

 eyepiece, as no doubt its chief, though not its 

 only, use will be to serve a demonstrator to 

 instruct a student. 



The new eyepiece slips into the draw-tube of 

 the microscope like an ordinary eyepiece. The 

 field of view is common to both eyepieces, and 

 contains a pointer which either observer can direct 



