Organic Bemains, Crystals, and Artificial Objects. 103 



known as "M. Nobert's" afford another example of 



this process of machine-ruling. ' ' As at present sup- 



plied, they consist of a slip of glass 



(No. 1 0),on which, by careful observation 



with the naked eye, may be observed 



what looks like a single shadowy line, 



sparkling in bright light with the play of 



prismatic colours. By moderate power 



(fifty diameters), with good glass and 



careful illumination, twenty distinct 



bands may be counted (that is, in the 



breadth and not the length of the line) ; 



the finer, however, very shadowy and 



evanescent. With 100 diameters, the 



four coarsest should be shown in distinct 



lines ; as the power is increased, more and more of the 



bands will be resolved, till with 500 diameters, the 



separate lines composing all but one or two of the finest 



of the bands should be clearly seen"* (No. 11). 



No.lO. Nobert's 

 Test-Lines. 



No. 11. 



It is calculated that the coarsest lines are about 

 the thirteen-thousandth of an inch from each other, 

 and the finest the eighty thousandth. 



Of microscopic writing, some remarkable speci- 

 mens were shown at the International Exhibition of 



* From an article on "Nobert's Test-Lines," by Mr. Tuffea 

 West, F.L.S., in " Recreative Science," vol. i. 



