1883.] 



MICROSCOPICAL JOURNAL. 



29 



the ordinary demands of original 

 investigation. For example, why 

 should a physician, who uses the mi- 

 croscope in his practice, pay $35.00 

 for an objective, while the $15.00 one 

 will answer his purpose just as well ? 

 Why should a student in a sea-side 

 laboratory, or i 1 a medical college 

 even, use anything better than the 

 cheaper lens ? As a matter of fact 

 and experience, the adjustment for 

 varying thicknesses of cover is useless 

 for their purposes. The only ques- 

 tion that remains is whether the cor- 

 rection objectives are not better made 

 than the others. We are inclined to 

 believe not. Too many people judge 

 of the quality of an objective by its 

 price, and for this reason there is an 

 unfortunate opinion current that cheap 

 American objectives are very inferior, 

 and not worth buying. This is not 

 true. One ^objective belonging to 

 the writer, which he uses almost ex- 

 clusively in his studies, has a collar- 

 adjustment, but probably the position 

 of the collar is not changed once in 

 six months for any purpose, and 

 surely it never is in the course of 

 observation. No doubt many others 

 have the same experience. How 

 many who use the microscope use the 

 collar- adjustment except for very 

 special kinds of work, such as the 

 study of diatoms, for example ? Very 

 few. The German investigators have 

 not found any difficulty in conducting 

 ver}' delicate examinations by means 

 of objectives of low price ; it is ab- 

 surd to say that our medical students, 

 whose requirements of a microscope 

 are very small at the best, should buy 

 more costly and better ones. It is de- 

 manding too great an expenditure of 

 money for the instrument, and thus 

 deters many a graduate from carrying 

 on his histological or pathological 

 studies. It is with regret that we 

 see German and French objectives 

 and microscopes finding their way 

 into our college laboratories where 

 only American instruments should be. 

 Yet it is clear that the former are, for 

 some reasons, used in preference ; 



and we have no doubt they will be, 

 until our manufacturers make some 

 efforts to demonstrate the equal excel- 

 lence of their cheaper grades of in- 

 struments. 



In touching upon the theoretical 

 part of this subject, we are pleased to 

 say that the grounds for a thorough and 

 satisfactory discussion of the problem 

 are found in the Abbe theory of mi- 

 croscopical vision, and have been 

 already well set forth by Dr. L. Dippel 

 in-the Zeitsch. fiir Instrumentcnkunde. 

 We shall avail ourselves of the pub- 

 lished article of Dr. Dippel in pre- 

 senting the matter as regards the 

 application of the collar-correction to 

 objectives of homogeneous immersion; 

 but first it may be desirable to refer to 

 its advantages in the case of dry 

 objectives of considerable angular 

 aperture. In the case of such ob- 

 jectives there can be no question of 

 the great importance of a correcting 

 collar, for when using oblique light, 

 only a portion of an annular zone of 

 the objective is used, and it is impor- 

 tant to correct the lens for the partic- 

 ular angle of illumination employed, 

 even at the sacrifice of the best cor- 

 rection of the objective as a whole. 

 It may be said that the importance of 

 correction-adjustment increases with 

 dry objectives as the angular aperture 

 increases. 



In the case of homogeneous immer- 

 sion lenses, however, the advantages 

 are not so evident. There are, in- 

 deed, certain benefits to be derived 

 from the adjustment, but these are so 

 slight that they cannot usually be re- 

 garded as worthy of much attention. 

 It enables fluids of slightly different 

 refractive power to be employed. 

 Some makers even apply such a range 

 of correction that the lenses will work 

 in any fluid from water to oil of cedar. 

 We cannot conceive how such objec- 

 tives can possess any special merits, 

 since the great range in the adjust- 

 ment must, it would seem, destroy the 

 correction of the objective for all fluids 

 but the one for which it was especially 

 constructed. 



