296 ACCESSOKY APPARATUS 



centres it, the differences in latitude and longitude may be noted, and 

 will give the constants for the correction which must be added to 

 or subtracted from the figures given by the sender. 



Mr. Nelson has made some very practical suggestions touching 

 the improvement of finders. He suggests, what we heartily accord 

 with 



1. That the stage-stop shall be always on the left hand of the 

 stage. 



2. That the zero of the horizontal graduation shall be on the left 

 hand of the scale. 



3. That the zero of the vertical graduation shall be on the top of 

 the scale. 



4. That when the finder is placed to 0, 0, a spot marked on the 

 bottom edge of a 3 x 1 inch brass template two inches from the stop 

 shall be in the optic axis of the instrument. In other words, the 

 latitude and longitude of the centre of a 3 x 1 inch glass slip shall 

 be 50, 50. 



5. That the division shall be in T ^ths of an inch, and the scales 

 one inch long. 



If these very simple suggestions were adopted generally, an object 

 found on one microscope could be easily found on any other. This, 

 like the ' Society's screw' for object-glasses and a universal sub-stage 

 fitting, deserves, in the interests of international microscopy, the 

 consideration of opticians. 



In practical ' logging ' the use of a hand lens will enable the ob- 

 server to read by estimation very accurately ; half a division can be 

 very approximately judged of, and this is as close as will be required 

 with the highest powers. We have found, for very delicate work, 

 that we could log with advantage between the divisions, thus : say 

 ' long. 41 ;' but if slightly over, but not an estimated half, * 41 + ; ' if 

 half, ' 41^ ; ' if more than this, but less than 42, it is logged ' 42.' 

 For logging purposes the lens we recommend is one of Zeiss's ' loups,' 

 magnifying six diameters. They are admirable instruments, and 

 are furnished with a handle, which may be used or riot at the w r ill of 

 the worker. 



The other finder we desire to consider is called after its inventor, 

 and is known, as ' Maltwood's finder.' 1 



It consists of a micro-photograph, one square inch in size, divided 

 into 2,500 little squares, so that each is ^th inch square. Each 

 square contains two numbers, one indicating the latitude and one 

 the longitude. To log any object the slide containing the object 

 must be removed and the slip holding the micro- photograph substi- 

 tuted for it ; then the figure in the square which most nearly agrees 

 with the centre of the field is noted. Of course, both the object and 

 the Maltwood finder must be carefully made to abut against the 

 stop. 



There are two drawbacks to this finder. 



1. The divisions are not fine enough, so that it is only suitable 

 for low powers. 



2. The removal of the slide, and its substitution by the Maltwood 



1 Trans, of the Micro. Soc. new series, vol. vi. 1858, p. 59. 



