MEMORANDA. 153 



Mr. Bailey alludes to the possibility of making the move- 

 able stage its own indicator, but the old and ever obstinate 

 vice of "selfishness" comes in his way, and he discards the 

 idea in favour of his paper instrument, forgetting, as it seems 

 to me, that his own very ingenious contrivance, the moveable 

 centrepiece, is easily applicable to the stage. I would sug- 

 gest, then, that if makers would rule their stages with the 

 required vertical and horizontal scales of 50ths or lOOths of 

 an inch, agreeing to adopt an uniform given distance from the 

 middle for the commencement of each scale, and would 

 supply an ivory disk, perforated in the centre for adjusting 

 purposes, much would be done towards the attainment of a 

 good " Universal Indicator." One trifling difficulty remains, 

 it is this : the disk (as will be found on experiment) should 

 not exceed 8-lOths of an inch in diameter, and the central 

 apertures of microscopes are usually much larger than this. 

 Tt)e simple remedy would be to furnish a metal or bone 

 collar, which would fit the aperture, and remain in it while 

 employing the finder. It strikes me, too, that as the scale 

 divisions of less than 50ths of an inch may be puzzling to any 

 but sharp eyes, and as finer divisions would certainly be 

 advantageous, larger ruling might be adopted, and a small 

 vernier made to slide in a groove by the side of each scale. — 

 '^1'homas Edward Amvot, Diss, Norfolk. 



On Micrometers applied to microscopes. — The ordinary 

 stage-micrometer, as ccmstructed by the best English and 

 foreign opticians, cannot be directly applied to the measure- 

 ment of very minute objects. Although it can be procured at 

 a moderate price, and with divisions beautifully ruled on 

 glass at intervals of 1-lOOth of a millimetre apart, the scale 

 is far too coarse for the use of the histologist ; and it is usually 

 quite impossible, in examining certain objects under high 

 magnifying powers, to bring their edges into proper focus 

 while the ruled lines of the scale continue tolerably defined. 



The eyepiece-micrometer, consisting of a scale ruled on 

 glass, and inserted in the stop or diaphragm of the ordinary 

 negative eyepiece, is a very convenient instrument, enabling 

 the observer, when using a magnifying power of 500 or 600 

 diameters, to estimate spaces of about l-200th or l-300th of 

 a millimetre with tolerable precision, in favourable circum- 

 stances. But the breadth of the lines on the best ruled eye- 

 piece-scale is so considerable, and the shadows caused by 

 their channels so perplexing, even when the illumination is 

 carefully managed, that, where extreme accuracy is requiied, 

 other apparatus must be employed. 



The cobweb screw-micrometer, when well constructed, is a 

 far more perfect instrument ; but, as Mr. Quekett remarks, 



