INDICATOR FOR MICROSCOPES. 57 



centre of the field of the microscope, the horizontal guide-line 

 upon the slides shall pass through the same numbers on two 

 vertical axes of the Indicator as remote from each other as 

 possible. In some positions of the slide the axes A and B' 

 can be used for this purpose ; in others A, and A', or B, and 

 B' must be employed. 



The horizontal line of the slide being arranged, as just 

 directed, it will be found that at least one of the vertical guide- 

 lines of the slide will intersect the horizontal graduation. By 

 observing now the numbers at which the guide-lines respec- 

 tively stand, the record can be made. Suppose, for example, 

 that the horizontal guide-line ruled upon the slide intersects 

 the verticals of the Indicator at 43, while the right hand ver- 

 tical of the slide cuts the horizontal series of numbers of the 

 Indicator at 75 ; the entry to be made for this object in the 

 register would be written thus ff'; and whenever that particular 

 object is to be found either by the same Indicator or aj}y other 

 copy of it, if the slide is placed at these numbers, and the In- 

 dicator is properly centred, the object must be in the field of 

 view. In the same manner any number of objects can be re- 

 gistered or found. If the slide happens to be so placed that 

 both of its verticals intersect the graduated portions of the 

 horizontal line C, D, the position of either one of them can 

 be recorded at will. 



If a guide-line upon the slide falls between two divisions 

 of either scale, the fraction of the division may be estimated 

 with sufficient accuracy by the eye or a hand-magnifier and 



25-' 



entered in the register. Thus the recorded position ^, 

 would mean that the vertical lines of the Indicator were inter- 

 sected at l-8th of a division of the scale beyond 25, while the 

 vertical guide-line of the slide passed l-4th of a division 

 beyond the number 34 of the horizontal scale, as nearly as 

 could be estimated. 



It is convenient to let the lower edge of the glass slide rest 

 against a straight-edged guide-piece, which can be moved 

 parallel to the horizontal line of the Indicator. By pushing 

 the slide along this edge, all the objects on the same horizontal 

 line can be found without changing the position of the guide- 

 piece. By moving the guide-piece a little forwards or back- 

 wards another sweep across the slide may be made, and so on 

 until every object of interest is found. 



By following the directions above given it will be found 

 that the recording or finding of objects by means of the Indi- 

 cator is very easily performed, and scarcely requires the time 

 which has been employed in describing the method. It is 



