172 Transactions of the Boy al Microscopical Society. 
difference in the refraction of glass and of balsam, to utterly 
obliterate the work. 
Mr. Eogers may be surprised to be informed that if he work with 
a diamond at all, it must be one which acts precisely as a cabinet- 
maker's plane acts when smoothing a board, and producing coiled 
glass shavings. The late Mr. Farrants tried, and the writer of 
these lines has tried, in vain to mount the glass spirals. 
Micrometers are sometimes ruled with a knife-edged diamond as 
well as with other cutting diamonds, and with bort, but they are 
then filled in with black, covered with balsam, and fixed down. 
When Mr. Eogers talks about what he designates as the grain 
of the glass (which he suggests may be the efiect of the polishing), 
and which he says prevents his ruling both ways, Mr. Eogers may 
reflect upon the fact that English stage-micrometers are all ruled 
both ways. 
A great patron of microscopology, Frank Crisp, Esq., of 
London, has a diamond engraving of the Lord's Prayer, in which 
the letters are smaller than the two hundred and ten millionth part 
of a square inch, at which size over fifty-nine [69] Bibles would be 
required to cover an inch ; and Mr. Crisp has also several others, the 
largest of which would fill an inch with a thousand letters. Some 
of these are blacked in and balsamed down, while others are not 
blacked in or balsamed down, and are all produced by ruling straight 
and curved lines, intersecting each other in every direction, and not 
one has a broken or jagged edged line. 
Mr. E. Wheeler, of Tollington Eoad, London, last year for- 
warded to the present writer a set of bands, after Nobert, to repair 
and remount, so as to obviate the effect of an accident, which lines, 
fourteen years after being ruled without a screw, and mounted dry, 
were still perfect in their gouge -like cut. 
With regard to what Mr. Eogers calls the " periodicity of errors," 
^nd which that gentleman in emphatic italics attributes not to the 
screw itself but to the mounting of the screw, possibly he will con- 
firm the experience of others if he take the head off the screw and 
naake arrangements to reunite them in different positions in rela- 
tion to each other, so that zero on the head would be coincident 
with different parts of the periphery of the screw at different times, 
and rule a set of lines with the screw and head in each of the dif- 
ferent relative positions, he would possibly find the periodicity of 
errors would be exactly the same, but the locality would be shifted ; 
this crucial test proving the fault to be in the screw. 
That lines cannot be ruled fine enough upon glass, but can be 
etched by the fumes of hydrofluoric acid, is a statement which is 
somewhat curious, because the fumes etch laterally — i.e., not only 
straight through the glass which has been exposed, but underneath 
the protecting wax on either side of the line — thus widening the 
