100 THE MICROSCOPE AND ITS REVELATIONS. 



as by the use of the Erector ( 84) or of the Nose-piece, no special means 

 are required; since, when the object has been found by a low power, and 

 brought into the centre of the field, it is rightly placed for examination 

 by any other Objective. Even this slight trouble, however, may be 

 saved by the adoption of more special methods; among the simplest of 

 which is marking the position of the object on the surface of the thin 

 glass which covers it. The readiest mode of doing this, when the object 

 is large enough to be distinguished by the naked eye or under the 

 Simple Microscope, is to make a small ring round it with a fine cameFs- 

 hair pencil dipped in Asphalte, or Brunswick- black (Indian ink being 

 objectionable, as liable to be washed off when water-immersion Objectives 

 are in use); but when the object is not thus visible, the slide must be 

 laid in position on the stage, the object ' found ' in the Microscope, the 

 Condenser adjusted to give a bright and defined circle of light, and then, 

 the Microscope-body being withdrawn, the black ring is to be marked 

 around the illuminated spot. This method, however, has the disadvan- 

 tage of concealing any other objects that may lie in close proximity to 

 the one around which the circle is drawn ; and recourse must be had in 

 such cases to some other plan. The Mechanical Stage may be easily 

 turned to account as & finder, by engraving upon it two scales, horizontal 

 and vertical, by which the object-platform may be exactly set to any 

 desired position; this platform being itself provided with a removable 

 * stop/ against which the glass slide (resting on its lower edge) may so 

 abut, as always to occupy the same place on the platform. Now sup- 

 posing an observer to be examining a newly-mounted slide, containing 

 any object which he is likely to wish to find on some future occasion, he 

 first lays the slide on the object-platform, with its lower edge resting on 

 the ledge, and its end abutting against the lateral stop, and brings the 

 object-platform itself to the zero of the scales; then, whenever, on 

 moving the slide by the traversing action, he meets with any particular 

 form worthy of note, he reads off its position upon the two scales, and 

 records it in any convenient mode. The scale may be divided to 50ths 

 of an inch, and each of these spaces may be again halved by the eye; and 



26 



ftie record may perhaps be best made thus, Triceratium favus - the 



18 



upper number marking the ' latitude ' of the object on the vertical scale, 

 and the lower its * longitude ' on the horizontal. Whenever the Micro- 

 scopist may wish again to bring this object under examination, he has 

 merely to lay the slide in the same positon on the platform, and to 

 adjust the platform by its scales according to the recorded numbers. 1 

 The 'finder' most commonly used is that invented by Mr. Maltwood, 8 

 which consists of a glass slide 3 inches by 1J inch, on which is photographed 

 a scale that occupies a square inch and is divided by horizontal and 

 vertical lines at l-50th of an inch apart into 2,500 squares, each of which 

 contains two numbers, one marking its ' latitude' or place in the vertical 

 series, and the other its < longitude ' or place in the horizontal series. 



1 This plan, first suggested by Mr. Okeden, might be adopted with so little 

 trouble or expense in every Microscope possessed of a Mechanical stage, that it 

 would be very desirable for every such Microscope to be furnished with these 

 graduated scales. If the different Makers would agree to use the l-50th inch 

 scale, Observers at a distance from one another, who might wish to examine each 

 other's objects, would have no difficulty in finding them by the record of their 

 positions accompanying each slide. 



2 ' Transactions of the Microscopical Society," N. S., Vol. vi, (1858), p. 59. 



