On the Use of the Noherfs Plate. 29 



done. But if instead, the blackened brass teeth which serve to 

 record the movements of the cobweb be used simply as reference 

 points to enable the eye to keep its place in counting across the 

 band, a little practice will soon enable the observer to count the 

 whole band with certainty and precision. The micrometer not 

 being touched, there will be no tremor. It will of course be under- 

 stood that a httle strip of brass furnished with fine teeth, gummed 

 to the diaphragm of an ordinary eye-piece, will answer the same 

 purpose. A glass eye -piece micrometer will be found to impair 

 definition too much. After a little experience, however, even the 

 specks on the ordinary eye-piece and on the plate itself can be em- 

 ployed as reference points, and will render a successful count fea- 

 sible, though not so easy as with the help of the economical con- 

 trivance I have described. Of course it is best in any case to 

 begin with practice on the lower bands. 



Having acquired the degree of skill necessary to enable him 

 with such help to count a number of fine hues without losing his 

 place, the conscientious student of the Nobert's plate has not, how- 

 ever, disposed of all the difiiculties in his path. With such oblique 

 light as is necessary for the resolution of all the higher bands with 

 our present objectives, he will find a certain number of spurious 

 lines on both sides of the band, and he will have to learn some 

 method of determining where to begin and where to end his count. 

 Nothing gave me greater difficulty in my earlier investigations of 

 the plate, and I hope, therefore, that a short account of the prac- 

 tical results at which I arrived may prove of service to other 

 microscopists. 



If one of the higher bands of the plate be examined by an 

 objective capable of resolving it, while it is illuminated by a pencil 

 of insufficient obliquity, a mere tint or shade of the width of the 

 band will be observed; with more obhque illumination a wavy 

 irregular aj)pearance comes into view ; increasing the obliquity of 

 the pencil still further, a series of lines make their appearance, 

 occupying about the width of the band, but fewer in number than 

 the real lines; finally, when the degree of obhquity necessary for 

 actual resolution is attained, the true lines start into view, accom- 

 panied, however, by certain spurious hues on the margins of the 

 bands. Four appearances of the bands are thus indicated, viz. a 

 mere tint or shade, a wavy irregular appearance, spurious lines 

 occupying the place of the band, and true lines with spurious lines 

 on the margins. 



The last two of these appearances requu^e consideration. 



A. Sjnirious Lines occupjing the Place of the Band. — These are 

 generally clean smooth lines, quite sharp, and much narrower than 

 the apparent interspaces. They are well calculated to deceive the 

 unwary, as they have often done. On a count, however, they wiU 



