ON A UNIVERSAL INDICATOR FOR MICROSCOPES. 55 



0)1 a Universal Indicator for Microscopes. By J. VV". 



Bailey. 



In the ' Quarterly Journal of Microscopical Science,' vol. i, 

 p. 34, an ingenious contrivance for registering the position of 

 microscopic objects is described by Mr. Tyrrel ; a modifica- 

 tion of this, by Mr. Aymot, is given in a subsequent number 

 (1. c, vol. i. p. 301) ; and a still better arrangement for the 

 same purpose, suggested by Mr, Brodie and applied by Mr. 

 Okeden to his microscope, is described at p. 166 of volume 

 iii. of the same work. The last mentioned device can scarcely 

 be improved upon for convenience ; but there is one defect 

 which is inherent to all these inventions, viz., that they are 

 essentially selfish contrivances, of no use to any one but the 

 owner of the particular instrument to which they may be 

 attached. 



The object of the instrument I propose is more comprehen- 

 sive than that of the " Finders" above alluded to, being no 

 less than to make a Universal Indicator, by means of which an 

 observer can so register the position of any number of objects 

 mounted upon slides, that when tliese are sent to a distant 

 correspondent the latter may be able by means of the Indicator 

 to find at will any of these objects, as easily as if he had the 

 identical microscope and " Finder " by which they were at first 

 I'ecorded. If such a mode of recording the position of objects 

 can be generally adopted that when the register is once made, 

 the record and the objects shall then be entirely independent 

 of the original instrument and observer, and applicable to any 

 microscope, it will tend to promote science not only by facili- 

 tating the interchange of specimens among naturalists, but it 

 will give to each observer's collection, when properly regis- 

 tered, a permanent scientific value and utility which it could 

 have in no other manner. 



The plan I have adopted is to make upon an engraved card 

 what may be considered as a transferable stage, having guide- 

 lines, by means of which the centie of the field of view of 

 the microscope, and the position of a slide when any object 

 upon it occupies this centre, may be given. 



Plate V. shows the Indicator complete. The centre of the 

 field of view corresponds to the intersection of the horizontal 

 line C, D, with the vertical line E, F, On the right and left 

 hand of this centre the vertical axes B and A' are placed at 

 distances of 4-5ths of an inch, and the axes A and B' are 

 similarly placed at the distances of 6-5ths of an inch from 

 the centre. 



