1893.] MICKOSCOPICAL JOURNAL. 171 



Note on Collar Correction. 



By S. G. shanks, M. D., 



ALBANY, N. Y. 



There is a certain but not a short cut to the art of managing a 

 correction coHar. Unfortunately, no simple rules can be formu- 

 lated that will fit all objectives, objects, and eyes, but, as in every 

 other sort of high-class work, the capable and willing can and do 

 surmount the difficulties and enjoy hne definition, while the lazy 

 one is waiting for the short cut to come to him. Every owner of 

 a good objective should also own a Moller test-plate or a series of 

 mounted diatoms of increasing difficulty of resolution. These 

 test objects are among the easiest to master, and attentive practice 

 over them, until the markings within the capacity of the objective 

 are resolved, will teach any one the value and general method of 

 using the collar. The following facts will be of assistance to the 

 student : 



The zero point of most objectives indicates the open point, i. r., 

 where the lens systems are most widely separated, and this is the 

 position of best correction for uncovered objects. A thin cover or 

 short tube requires the collar index near zero, i. e. , the open 

 point. xA. thick cover or an object low down in the balsam or a 

 long tube requires the collar moved somewhat away from zero, 

 /. ^., the systems nearer the closed point. 



Correcting by coma. — Find a small speck of dirt in the mount, 

 at about the level of the object, focus up and down, noticing the 

 hazy edge or coma surrounding the speck when out of focus. If 

 this is wider when the objective is above the focal point, turn the 

 collar toward zero. If the coma is wider when the objective is 

 lowered below the sharp focal point, turn the collar away from 

 zero. Adjust until the coma is uniform within and without the 

 focal point. 



Correcting by color. — Find a speck of dirt as above, focus sharp- 

 ly. If the edge of the speck shows a yellowish tinge or halo, turn 

 the collar away from zero. If the edge shows a bluish tinge, turn 

 the collar toward zero. When correct the high points or corners 

 of the speck should be ruby red and the lower edge show a fine 

 apple-green halo, narrow but distinct. 



Correcting by coma and by color as above are useful when the 

 elements of the object are very delicate and have no well-defined 

 edges or markings. Diatoms or fibres are the easiest for practice 

 and may be used directly. A diatom under a well-corrected ob- 

 jective (not an apochromatic) will appear flat and crisp, the high 

 parts, edges or midrib, will be tinged with a beautiful ruby-red 

 color, and the shadows will have a delicate, apple-green tint. 

 The majority of objectives being slightly undercorrected for color, 

 will exhibit these tints when at the point of best correction. 



Neither coma nor color will be distinctly seen at first by the 

 novice, but after some practice they will become plain enough. 



