EMARKS ON FOREGOING TABLES.
It will be seen from these tables that there is no advantage to be derived, other than that of increased magnification, by limit of useful magnification. For micrography the " projection " eyepieces using an eyepiece of high magnifying power ; it simply magnifies the image pro duced by the objective without adding to the detail—in fact, any imperfections clue to defects in the objective are at the same time magnified and made more manifest. Of course, if the lens is a good one the image formed by it can be magnified very considerably without any appreciable de fect, the amount that it can be so magni fied being shown under the head of highest are usually employed ; these differ from the ordinary form in that they are com posed of a collective lens and a compound system (Fig. 693). A diaphragm is placed between these lenses to limit the field, and the compound system is moved by re volving the eyepiece cap until a sharp image of the diaphragm is thrown on the screen ; a scale is provided to register the proper position for any screen dis tance. The designation of these eyepieces
denotes the magnifications which would be obtained by using these lenses alone. The eyepieces 2 and 4 give increased mag nifications of 2 and 4 with a 160 nun. (6 in.) tube, or 3 and 6 with a 250 nun.
(10 tube. Compensating eyepieces (Fig. 099) have already been mentioned ; they are for use with apochromatic ob jectives only, but eyepieces are now made, with a sliding tube for correction, that can be used either with the ordinary achromatic objectives or with the apo chromatics. The denomination of the fitted to the stand holding the lamp (Fig. 701). The bull's-eye condenser is used with low powers, and is placed between the lamp and the microscope. The collective lens of Zeiss with an iris diaphragm is au improvement on the ordinary condenser, as it transmits only a circle of light sufficiently large to be utilised by the ob compensation eyepieces is also in cordance with their magnifying powers, 2, 4, 6, 8, 12, and 18.