1897] MICROSCOPICAL JOURNAL. 97 



PRACTICAL HINTS. 

 By R. H. ward, M. D. 



TROY, N. Y. 



A Simple Expedient in Focusing. I have just noticed 

 that one intended sug-g'estion, which is ])erhai)s curious 

 enoug-h to be worth noticing- separately, was inadvertently 

 omitted in putting in order my article (jn "Focusing Up- 

 ward" in a former No. of Thio Mickoscopk. In the method 

 there recommended as the only safe one for the irlexper- 

 ienced, and the best one for all, of looking- horizontally 

 through, between the objective and the slide, until the 

 lens is near the slide without touching it, there is often 

 difficulty, in certain arrangements of the microscope and 

 the light, requiring- lig-ht to be thrown throug-h by a hand 

 mirror, or a brig-ht background to be presented by holding 

 up, in suitable position and lig-ht, a piece of white paper or 

 card. In such cases it is often very easy to trace the de- 

 scent of the lens by looking obliquely downward'and view- 

 ing- the reflection of its lower face from the surface of the 

 slide. This method, which is familiarly and safely used 

 by the expert, is however a critical one, and excessively 

 dang-erous to the rash and inexperienced, especially if not 

 thoroughly familiar with optical principles and appear- 

 ances. The working- distance of the objective is not 

 shown directly, as in the former case, but obliquely and it 

 may easily be misjudged ; and the end of the mounting- of 

 the objective is not always what or where it seems. There 

 are of course, moreover, four reflections in dry mounts, 

 from the top and bottom each of the cover-g-lass and the 

 •^lide, though two of these are naturally obliterated by 

 "medium" in other mounts, and the deeper reflections are 

 not usually distinct enoug-h to mislead, even if noticed at 

 all. n^his method, however, should not be used by beg-in- 

 ners, nor ever with olijectives or slides that are not the 

 jM-operty of the manipulator; as a slight misunderstand- 

 ing would cause a fatal accident to slide or objective, if not 

 to both. 



