106 APPENDAGES TO THE MICROSCOPE. 



against the stop ; and when some particular object, whose place it 

 is desired to record, has been brought into the field of view, the 

 object-slide being removed and the Finder laid down in its place, 

 the numbers of the square then in the field are to be read off and 

 recorded. To find that object again at any time, the Finder is 

 to be laid in its place on the Stage, and the stage moved so as to 

 bring the recorded number into view ; and the object-slide being 

 then substituted for the Finder, the desired object will present 

 itself in the field. As care is taken in the production of each 

 instrument that the scale shall be at an exact distance from the 

 bottom and left-hand end of the glass slide, the Microscopist may 

 thus enable any other observer provided with a similar Finder 

 to bring into view any desired object, by informing him of the 

 numbers that mark its latitude and longitude. These numbers 

 may either be marked upon the object-slide itself, or recorded in 

 a separate list.* 



78. Diaphragm. — The Stage of every Microscope should be pro- 

 vided with some meansof regulating the amount of light sent upwards 

 from the Mirror through transparent objects under examination. This 

 is usually accomplished by means of a Diaphragm-plate, perforated 

 by apertures of different sizes, which is pivoted to a removable 

 fitting attached to the underside of the Stage (Fig. 35), in such a 

 manner that by rotating the plate either of the apertures can be 

 brought into the optic axis of the instrument. This plate should 

 be always at least half an inch below tbe object, since it is otherwise 

 comparatively inoperative. The largest of its apertures should be 

 made to carry a ground-glass (so fitted as to be removable at plea- 

 sure), the use of which is to diffuse a soft and equable light over 

 the field when large Transparent objects (such as Sections of Wood), 

 are under examination ; between the smallest and the largest aper- 

 ture there should be an imperforated space, to serve as a dark 

 back-ground for Opaque objects. The Diaphragm-plate itself, the 



* Other "finders" have been suggested in the pages of the "Quart. 

 Micros. Journal," by Mr. J. Tyrrell, Mr. E. G. Wright, Mr. T. E. Amyot, 

 and Mr. Bridgman, at pp. 234 and 302-304 of Vol. i. ; by Prof. Bailey. 

 Mr. Amyot, and Mr. Hodgson, at pp. 55, 151, 209, and 243 of Vol. iv. ; by 

 Mr. Farrants, in "Trans, of Micros. Soc." Vol. v. p. 88; and by the 

 Committee appointed for the purpose, in the same volume, p. 95. Some 

 of these have been superseded by Maltwood's Finder, but as this cannot 

 be conveniently used except with a Mechanical Stage, those who do not 

 possess that convenience must have recourse to such of the above- 

 mentioned plans as they may find most suitable to their respective 

 purposes. — Some of these methods only enable the Microscopist to 

 " find " his own object, whilst others enable him to indicate it to any 

 other observer. A very simple method of the former kind, applicable to 

 Stages fitted with side-springs for holding the slides (Figs. 32, 41), has 

 been pointed out to the Author by Mr. Moginie. If a small nick be 

 filed in the inner edge of each spring at about the middle of its length, 

 it is easy, when an object has been brought into position, to make two 

 small ink dots upon the paper cover of the slide, by a fine pen inserted 

 into each nick ; and whenever the two dots are brought again into their 

 corresponding nicks, the object will be found hi the field. 



