THE MICROSCOPE. 



jectives, the front lens moves and the inner systems are fixed. In 

 either case the movement is produced by a milled collar around the 

 body of the objective. This collar is generally graduated so that 

 its position can be recorded for future reference. In a cheaper 

 grade of objectives the adjustment is made by a screw-movement of 

 the outer tube. This rotates the front lens, and unless well made it 

 is apt to be decentered. However, if well made, it forms a very 

 efficient and cheap adjustment. 



While it is true that this adjustment corrects the aberrations 

 produced by the cover-glass, it also compensates for the difference 

 in length of the bodies of different microscope stands, and the dif- 

 ferent fluids used for immersion with objectives intended for that 

 mode of use. 



This matter of collar correction does not seem to receive the 

 attention which it deserves, and among the younger microscopists 

 it seems to be overlooked entirely. As so little has been written on 

 this subject, I will give a few simple directions. 



Every objective has a certain color with which it shows best, 

 and there is probably no object better adapted to the purpose of 

 determining this color than a well marked Podura scale. Although 

 this is a most excellent test-object, it has nothing to do with dia- 

 toms, and the young microscopist need have no fear of being called 

 a " diatomaniac " from the possession of one. When a good scale 

 is once obtained, great care should be taken to keep it dry, for when 

 wet it is of no use. 



Now, by examining this scale with a first-class % or higher 

 power of medium or wide aperture it will be seen that the "exclama- 

 tion marks" are more or less colored. Pay no attention to this at 

 first, but carefully turn the collar back and forth until the marks 

 appear sharpest and smallest. That will be the point of best correc- 

 tion, and now the color of the markings should be noticed. Having 

 carefully determined the exact tint of best correction, throw the ob- 

 jective a little out of proper adjustment by turning the collar towards 

 open point or zero. This over corrects it and at the same time 

 notice the change in color. The markings seem to expand, becom- 

 ing hazy and not at all sharp. Now, turn the collar towards closed 

 until the point of best correction is passed; here the same thing is 

 seen in regard to expansion and haziness, but a different tint seems 

 to make its appearance. By attending very closely to this color 



