206 THE USE OF THE MICROSCOPE 



by wide cones. The writer does not know how it is with 

 other biological workers, but when he has to work with a 

 microscope provided with this traversing and rotating iris, 

 he puts balsam on the cogwheel and rack to stop the travers- 

 ing, and wires the milled handle to the stage to stop the 

 rotating. Then only he can work the iris with steadiness. 



Color Filter.— The writer thinks, now that artificial 

 light is mostly employed for microscopical work, that 

 nearly every microscope should be provided with daylight 

 glass, and also yellow-green glass (or the corresponding 

 Wratten gelatin films). Some means of regulating the 

 intensity of the light should be found in every microscope 

 box, and three or four grades of neutral, or blue-tinted, 

 or yellow-green glass would usually suffice. 



Sliding Bar. — English microscopes often have (or had) 

 a sliding bar on their square stage. This is usually absent 

 from the square stages of the continental microscopes. It 

 is useful for searching, with the microscope inclined, in 

 the absence of a mechanical stage, or of an object traverser; 

 and in such case, might perhaps well be on the standard 

 microscope with square stage. The writer has used the 

 Detto sliding bar with satisfaction for years. 



Monochromatic Objectives for Green and Violet. — Yon 

 Rohr calculated one high-power monochromatic objective for 

 a special wave length of ultra-violet light (in the cadmium 

 spectrum). We do not yet, however, have a monochroma- 

 tic objective for the bright-green line of the mercury- 

 lamp spectrum, which should be excellent for visual work 

 (Barnard). Objectives might also be made corrected for 

 the short range of the spectrum passed by a definite yellow- 

 green screen, used with a definite tungsten electric light. 

 Also, for the blue or violet light passed by another definite 

 screen, to be used for photography alone. Such a pair 

 of objectives would replace one apochromatic objective, 

 and need not have color corrections. The advantage 

 would be that those who did not want to photograph would 

 not have to take an objective also corrected for photo- 

 graphy, as are the present apochromatic objectives. Such 



