MEMORANDA. 243 



Dr. Robertson complains that in glass micrometers " the 

 breadth of the lines is so considerable, and the shadows caused 

 by their channels are so perplexing," that extreme accuracy is 

 unattainable with them. 



From his constantly speaking of millimeters, I conclude 

 that the micrometers he has used are of French manufacture, 

 some of which, as I have seen, are justly liable to his censure, 

 for they appear to have been ruled by a diamond which cuts, 

 or rather splits, the glass like that employed by glaziers. But 

 micrometers may be obtained from most of the makers in 

 London, in which tlie lines are only thick enough to be dis- 

 tinctly visible ; and the channds being filled with plumbago, 

 and having a cover cemented over them with Canada balsam, 

 cast no shadows when in the focus of the eye-glass, the shadows 

 which Dr. Robertson complains of, being most probably the 

 refraction of the prismatic edges of the channels. — George 

 Jackson, 30, Church Street^ Spitalfields. 



Mr. Amyot'!^ Fiiider.— You Were good cnough to insert a short 

 paper of mine on the " Finder or Indicator " in the last 

 number of the ' Microscopical Journal ; ' I have since had 

 some lithographed scales struck off for pasting on the face of 

 the wooden instrument, and liave had the pleasure of supply- 

 ing a considerable number of these to gentlemen unknown to 

 me, and to whom I have not had time to write full directions 

 for attaching them. Indeed it is only within the last few days 

 that I have hit upon a mode of effecting this with ease and 

 satisfaction to myself. If you would be good enough to insert 

 the few necessary directions which I have condensed to the 

 utmost, you would confer an additional obligation. 



Directions for attacking the Lithographic Paper Scales to the Wooden 

 Indicator, described at page 151 of the last Number of the Journal. 



1. Continue the lines of tlie four scales across the centre of the paper, 

 using a fine-pointed pencil. 



2. Perforate the central intersection of lines with a needle. 



3. Cover the face of the wooden instrument with a thin layer of smooth 

 paste, the bone disk being removed. 



4. Force the little brass pin out of the bone, and replace the disk on its 

 rabbet. 



5. Place the paper on the wood, inclosing the bone, and then holding 

 the instrument up to the light, bring the needle-hole exactly opposite to 

 that in the bone, and ascertain mth a lens that this is correctly done. 



6. Smooth the paper on tlie wood, and before jjutting aside to dry again 

 ascertain that the centres are correct, 



7. In about two hours remove the middle of the paper at the ring with 

 a sharp-pointed penknife. Take out the bone disk, replace the pin, and 

 trim the edge of the paper so neatly that the centre-piece may fall easily 

 in its place. 



Thomas E. Amvot, Diss, Norfolk. 



