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232
MICRO-PROJECTION WITH OCULARS
ICH. IX
§ 358. Micrometer ocular for demonstration. — It is so difficult for most students to understand the workings of the ocular micrometer, that it is of great help to them to use a micrometer ocular like fig. 130 to 131 on the projection microscope, then the object and micrometer lines can be projected together by suitably adjusting the eye-lens of the ocular. A stage micrometer might also be used as object and the students shown, all together, how to determine the ocular micrometer valuation (see Gage, The Microscope).
Oculir lo 2
FIG. 129. COMPENSATION OCULARS.
(From Zeiss' Catalogue, No. jo).
A section has been removed to show the construction. The numbers 2, 4, 8, /2, 18, indicate the magnification of each ocular (see § 357a, 391 a).
§ 359. Substage condensers. — The writers believe, from their experience and experiments in photometry under the different conditions, that it is better to use for illumination only the large condenser (fig. 121).
The use of a substage condenser is for either one of two purposes : (i) to enable the position of the object and the projection objective
The average increase in magnification given by the different oculars with the different objectives and screen distances shown in the table (§ 377) is as follows: Projection ocular X2 gives a magnification of ................... 1.99
*4 " ................... 3-^9
Compensation " X2 " ................... 2.05
Huygenian x4 " ................... 4.2 1
From these figures it is seen that the increase in magnification for projection work can be closely enough approximated by multiplying the image given by the objective alone by the number designating the ocular, i. e., 2 or 4.
If very precise results are desired, one must use a stage micrometer and proceed as described in § 391 a.