ACCESSORY APPARATUS. 101 



The slide, when in use, should rest upon the ledge of the stage of the 

 Microscope, and be made to abut against a stop about 1J inch from the 

 centre of the stage. In order to use this 'finder,' the Object-slide must 

 be laid upon the Stage in such a manner as to rest upon its ledge and to 

 abut 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 'Malt wood,' that the scale shall be at an exact dis- 

 tance from the bottom and left-hand end of the glass slide, the Micro- 

 scopist 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. l 



98. Diaphragms. Every Microscope should be provided with some 

 means of regulating the amount of light sent upwards from the Mirror 

 through transparent objects under examination. This is usually accom- 



Elished by means of a Diaphragm-plate, perforated by apertures of dif- 

 3rent sizes (the smallest of which should be no larger than a pin-hole), 

 and pivoted to a removable fitting attached to the under side of the 

 Stage, in such a manner that by rotating the plate, either of the aper- 

 tures can be brought into the optic axis of the instrument. The larg- 

 est of its apertures should be made to carry a ground-glass (so fitted as to 

 be removable at pleasure), the use of which is to diffuse a soft and equable 

 light over the field when large transparent objects are under examination 

 with a low power; while between the smallest and the largest aperture 

 there should be an unperforated space, to serve as a dark background for 

 Opaque objects. The edge of the Diaphragm-plate should be notched 

 at certain intervals, and a spring-catch fitted so as to drop into the 

 notches, in order that each aperture may be brought into its proper cen- 

 tral position. When the Diaphragm-plate is used to improve the defini- 

 tion of high powers, it loses much of its value if its aperture be not very 

 close to the under side of the object-slide; and any arrangement which sets 

 it at some distance beneath the stage is consequently objectionable. Its 

 best position is in the thickness of the stage, which, for receiving it, is 



1 The only drawback to the utility of the Malt wood finder lies in the fact that 

 a single square more than covers the field taken in by l-4th Objective with 

 the A eye-piece; so that with powers many times as great, the proportion of the 

 square viewed at once is so small, as to make it impossible to fix the place of the 

 object with any precision. To obviate this difficulty, Mr. W. Webb proposes a 

 finder ruled with lines only l-200th of an inch apart, so as to divide a square of 

 only 3-4th of an inch into 22,500 squares. As it would be impossible to mark dis- 

 tinguishing numerals within squares of such minuteness, he rules stronger lines 

 at intervals, so as to divide the whole area into ' blocks' of 100 squares in each; 

 and any individual square can be easily described (1.) by the block in which it lies, 

 and (2) by its position in that block. ("Jqurn. of Roy. Microsc. Soc.," Vol. iii., 



; of each 

 may be 



h Objective; and that, if thus brought into the 

 centre of its field," the object will lie within the field of any Objective of higher 

 power, provided the centering of the two be conformable. 



