i 4 6 THE MICROSCOPE. 



The former treated in a happy manner the Relation of Aperture to 

 Amplification in the Selection of a Series of Microscopic Objectives. 

 He discussed, from an optical and mathematical standpoint, the 

 microscopical outfit which shall enable us to see clearly all the de- 

 tails which, invisible to the unaided eye, are yet visible by the aid of 

 the microscope, the author keeping in mind this proposition: that it 

 is worse than useless to employ, in any given case, any more of 

 either aperture or amplification than is necessary for the work in 

 hand. Concerning the combinations required by theory, he says: "I 

 have selected, as a set of powers sufficient for all the work of any 

 microscopist, the following: 



One 4-inch objective of o.io N. A.= 12° air angle nearly. 



The first two to be dry-working objectives without cover correc- 

 tion, the third to be dry-working with cover correction, the fourth to 

 be a homogeneous immersion objective with cover correction, and all 

 to be of the highest possible finish, * * * and [the stand] to be fur- 

 nished with six eye-pieces, viz.: 2-inch, i-inch and ^-inch Huy- 

 ghenian and %, ]/■$, and %-inch solid." 



The paper referred to by Prof. Rogers was A Critical Study of 

 the Action of a Diamond in Ruling Lines on Glass. It was a mas- 

 terly paper, which should be studied by all students of the micro- 

 scope. The author believes "that the lines composing Nobert's 

 finest bands are produced by a single crystal of the ruling diamond, 

 whose ruling qualities improve with use. * * * If Nobert had 

 any so-called secret, I believe this to have been its substance." Con- 

 cerning smooth lines he said: "One would naturally suppose that a 

 line of the best quality would be produced by the stoppage of the 

 light under which it is viewed by the opaque groove which is cut by 

 the ruling diamond. Without doubt, this is the way in which lines 

 are generally formed. But it is not the only way in which they can 

 be produced. An examination under the microscope will reveal the 

 fact that, in some instances at least, a portion of the glass is actu- 

 ally removed from the groove cut by the diamond, and that the 

 minute particles of glass thus removed are sometimes laid up in 

 windrows beside the real line, as a plow turns up a furrow of soil.'' 



