COBRESPONDENCE. 237 



The whole matter is made to turn on Schleiden's meaniug, and then, 

 by assuming that that distinguished botanist refers only to certain 

 omissions in Link's plates, the question is begged most effectually. 



In spite of the contrary opinion held by my nameless critic, I 

 contend that Schleiden is only speaking of errors of interi^retation 

 either as omissions or commissions, and as such, I venture to think 

 that the question possesses interest at the present time, from the simi- 

 larity of some of Dr. Pigott's expressions to those made use of by 

 Link. 



I am, Sir, your obedient servant, 



B. Daydon Jackson. 



An Error. 



To the Editor of the ^Monthly Microscopical Joiiriud.'' 



Boston, Sept. 11, 1873. 

 Sir, — In copying the article from the ' Lens,' in September number, 

 p. 148, you have of course copied the typograjihical errors. If you 

 will in the next number ask your readers to substitute lens and lenses 

 for the words "base " and " bases" where they are printed in the paper, 

 you will oblige 



Yours respectfully, 



Charles Stodder. 



Overcoming the Difficulty of Working with Immersion 



Lenses. 



To the Editor of the ^Monthly Microscopical Journal.^ 



Liverpool, Sept 18, 1873. 

 Sir, — In this month's number of your magazine Mr. H. U. Jansou 

 describes what has been to him, and doubtless to many others, an im- 

 portant practical difficulty in the use of immersion objectives. He 

 says : — " It is a decided objection that the interposed drop of water 

 greatly prevents our judging of the actual distance of the outer lens 

 from the covering glass" over the object. Having hit upon a very 

 simple method of overcoming this difficulty, I will describe it. With 

 a fine camel's-hair pencil put a drop of clean water over the lower 

 lens of the object-glass, taking care to see that it adheres. Screw it 

 gently on the microscope, and rack downwards until, on looking across 

 between the slide and the object-glass, the drop of water appears 

 sufficiently flattened out, and the focal distance of the lens approxi- 

 mated to. Then sit down to the instrument. Apply the left forefinger 

 nail to the upper edge of the slide, immediately in front of the object- 

 glass, and raise it until it is felt to touch. The distance will appear 

 quite considerable. Then take off the finger from the slide, and, 

 looking through the instrument, focus downwards with the fine ad- 

 justment, occasionally raising the slide as before with the left fore- 

 finger until the object is seen to come into view. In tliis way, the 



