124 



THE AMERICAN MONTHLY 



[J"uly, 



will 



charge 



pro- 

 for 



One dollar the hundred 

 bahly be a fair extra 

 accurately measured covers. 



Purchasers generally do not know 

 hoAV the cover-glass is obtained. It 

 is a very difficult article to make. 

 The glass makers have nothing to 

 do with the cutting up into squares 

 and circles ; they simply assort it 

 by roughly estimating the thickness 

 by feeling with the lingers into 

 three grades, Nos. 1, 2 and 3. Of 

 course the division line between 1 

 and 2 is indehnite — some No. 2 

 will be thinner than some No. 0. It 

 is sent from the glass house in irre- 

 gular sheets, from 1 inch to 15 or 

 20 inches, superficial area. Each 

 sheet may vary in thickness in dif- 

 ferent parts. It goes into the hands 

 of the cutters, who usually pack 

 the squares or circles labelled with 

 the original numbers of the glass 

 maker, and of course subject to all 

 the variations that " R. B. jr.-' 

 found in his clumsy method of 

 measurement. Some dealers are 

 more careful about thickness, and 

 take a measurement of each sheet 

 of glass before cutting — and then 

 calling for example all from j-^to 

 to T-A-n No. 1 — with some similar 

 Nos. 0, 2 and 3. But 



1 q 



limit for 



as 



will easily be seen this does not 

 secure accuracy. Neither in prac- 

 tice, is there any need of it except 

 for very high power objectives — 

 such as from ^^^h to -^V^^- ^ ^"^o- 

 dern first-class water immersion 

 objectives from ^L-inch down, with 

 " adjustment," work satisfactorily 

 through covers of varying thickness 

 from^^l^ to y^. "Homogene- 

 ous immersion " objectives with- 

 out " adjustment " are said to work 

 equally well with any thickness of 

 cover. 



Formerly — 10 to more years ago 

 —English ^-inch and other foreign 

 objectives had so short working 

 distance, that their owners had to 



hunt for very thin cover-glass in 

 order to use them. Such objectives 

 are now behind the times, American 

 opticians produce one-tenths that 

 will readily work correctly through 



^^^ ' Carl Reddots. 



The Microscopical Examination 

 of Signatures.* 



BY PKOF. LESTEK CURTIS, A. B., M. D. 



We all know that every one who 

 writes acquires certain habits of 

 writing. He is accustomed to form 

 his letters in a certain way and at 

 pretty definite distances apart ; he 

 writes on the line, or above it, or 

 possibly below it ; he has habits of 

 neatness in writing, or his writing 

 is marred with blots or erasures. 

 He has certain habits of punctua- 

 tion or want of punctuation. He 

 may, also, have certain character- 

 istic errors in spelling, or his spell- 

 ing may be correct but formed on 

 some peculiar model. All these, 

 and many other things, make the 

 writing of every one characteristic, 

 and render it practically impossible 

 to imitate any large amount of 

 writing so successfully as to avoid 

 detection by one at all skilled in 

 the examination of such things. 

 Forgers, therefore, are accustomed 

 to confine their operations almost 

 exclusively to the imitation of sig- 

 natures, and these signatures are 

 usually direct copies. There are 

 several methods of procedure that 

 may be followed by forgers. The 

 oldest and the most common 

 of these is direct imitation with a 

 pen, as one would write after a 

 copy in a copy-book. But however 

 close this imitation may be, it is 

 never so close as to avoid some im- 

 perfection which can be detected 

 by care, particularly if the micros- 

 cope is used in the examination. 



* Read before the State Microscopical So 

 ciety of Illinois. 



