202 THE MICROSCOPE. 



my explanation of the course pursued by the rays of light in the 

 resolution of amphipleura pellncida, as set forth in the June issue of 

 this journal, "is untenable and contrary to all optic laws." 



So far from accepting his opinion and agreeing with this state- 

 ment, I shall now endeavor to show and prove that my explanation, 

 as given in the June issue, is the only one which is not ''untenable 

 and contrary to all optic laws!" 



Dr. Detmers states that under the conditions by which Prof. 

 Forbes obtained the resolution — and which were subsequently suc- 

 cessfully repeated by himself — the field had presented a yellowish 

 tinge. 



Now, I desire to have Dr. Detmers answer this question: In 

 accordance with what law of optics, does a ray of sunlight become 

 yellow, after having been internally reflected from a colorless, trans- 

 parent medium!! 



Had Dr. Detmers noticed the color of the field he would have 

 seen that the exact tinge depended upon the color of the metal of 

 which the front cell of the objective was made. That is to say, that 

 if the front cell were made of red brass, the field would have had a 

 dark yellow tinge; while if yellow brass or nickel it would have been 

 a lighter yellow or nearly white. 



Now, if the light were reflected from the edge of the drop of 

 immersion fluid — as claimed by Dr. Detmers — what causes the 

 change of color in the field when objectives of different colors of 

 brass are used? 



Yet Dr. Detmers positively states that such is the case. 



The chief objection which Dr. Detmers imposes upon my ex- 

 planation, seems to be that as the angles of incidence and reflection 

 are invariably equal, the reflecting surface of the front of the objec- 

 tive does not present the proper angle to reflect the light upon the 

 lower (internal) surface of the slide — where it is then reflected 

 through the object and into the objective. But the only evidence 

 presented in support of this point, is that the front of his objectives 

 are well polished, beautifully lacquered, etc., and present no holes to 

 reflect the light where wanted. Dr. Detmers seems to forget that 

 opticians are not in the habit of putting a "perfect finish" to the 

 fronts of their objectives, and I have never yet seen an objective 

 which would not radiate (diffuse) light from its front cell. If sun- 

 light be condensed upon the front of an objective, perpendicular to 



