72 Maliwood's Finder Supplemented. 



ffir the indications of his own instrument shall coincide with those 

 of the original observer by whom the marking was effected, conse- 

 qiiently it would often happen that " there was nothing in the field." 

 It is, hence, indispensable that he should have some means of de- 

 termining, not only whether there be any discrepancy, but that he 

 should be able to ascertain the exact amount and direction in which 

 they may differ. This can be effected in a very simple manner. 

 Take, for instance, a common gummed label and draw two hues 

 from one side to the other at right angles so that they cross in the 

 centre, and then at the point of intersection make a minute puncture 

 with a fine needle. Next, place the instrument in position, icith the 

 light central, and putting on the "Maltwood" bring the central 

 square into view, and let the centre of the square be as near in the 

 centre of the field as possible, so that if a mark were to be made 



1 25 I 



then it would appear thus, | •_ j Now, take a common glass 



slip and place it on the stage in the place of the finder, and having 

 wetted the label place it on the glass and move it about until the 

 puncture comes into the centre of the field. If this be accurately 

 done the two ought perfectly to coincide, and therefore when this 

 correspondence is tested under different instruments the variation of 

 distance and direction from where the puncture really appears and 

 where it ought to he, will show the different spots at which the same 

 object will be found in each instrument. A test-slide with the 

 marker s name accompanjdng slides sent for inspection will always 

 afford a " key " for then- successful examination ; or where parties are 

 in frequent communication, or interchanging objects, one might be 

 kept in store by each correspondent. 



The accuracy wdth which this central spot can be determined will, 

 of course, regulate the degree of perfection which may be attained, 

 hence it is desirable to have some mechanical means by which its 

 exactness shall be measured. A very simple pointer may be made 

 in a few minutes by almost any ordinary observer. From the B 

 eye-piece unscrew the upper portion so as to expose the stop in the 

 focus of the eye-lens. W ith a pair of compasses mark a circle on a 

 thin piece of stifi" card which, when cut out, wiU just fit firmly into 

 the tube of the eye-piece, then mark another circle a little larger 

 than the opening of the stop, and draw a fine through the centre 

 quite across. Next cut out the central portion and fasten down the 

 ring on to a piece of white paper by two pins just above the line, 

 and complete the line across from one to the other on the paper 

 on to which the ring is pinned. A point midway of this hne will 

 now be the exact centre of the ring, and it only remains to fasten 



