. The microscope and its revelations. both here and on the Continent, are a mistake for A very good way of cutting down a cone from a mirror is tohave the diaphragm fitted in the sub-stage, so that it can be madeto advance or recede from the object. The advantage thus gainedis that one aperture is made to do the duty of several. It alsopermits of careful adjustment. The iris diaphragms are so comparatively inexpensive, that theyhave superseded for general work and ordinary purposes all others ;but whatever diaphragm is used it should work easily. Iris dia-phragms work sometimes
. The microscope and its revelations. both here and on the Continent, are a mistake for A very good way of cutting down a cone from a mirror is tohave the diaphragm fitted in the sub-stage, so that it can be madeto advance or recede from the object. The advantage thus gainedis that one aperture is made to do the duty of several. It alsopermits of careful adjustment. The iris diaphragms are so comparatively inexpensive, that theyhave superseded for general work and ordinary purposes all others ;but whatever diaphragm is used it should work easily. Iris dia-phragms work sometimes so stiffly that the microscope may lie movedbefore the diaphragm. So. too. with the diaphragm wheels ; somerequire a pair of pliers before they can be rotated. This is easilyaccounted for when we examine the way in which they are fixed. Theusual method is to screw the wheel t the under side of the metal , if there are neither washers nor a shoulder to the screw, it is1 Quekett, Micro. Journ. vol. iv. p. 121 et 298 ACCESSORY APPARATUS more than probable that when the diaphragm is rotated it willscrew up ami jam. The purchaser may easily observe a matter ofthis kind. ( Minder diaphragms, which were invented in 1832 byC. Varley. arc much used . m rhe Continent; they are also often madeinto iris form-. Also diaphragms with a very minute circular holein the line of the optical axis are largely used just behind theobject-slip. These are employed with the mirror only (withoutcondenser) and with daylight alone. The object of this methodof illumination being to render very translucent objects visible byincreasing the si/.e of the black diffraction bands at their edges, itis. as before stated, of no use for critical work. Condensers for Sub-stage condenser is anabsolutely indispensable part of a complete microscope. Its valuecannot be overrated, for the ability of the best lenses to do theirbest work, even in the most skilful hands, is determined by it
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