214 



THE AMERICAN MONTHLY 



[November, 



Choosing Objectives. — A cor- 

 respondent in last month's issue asked 

 for some information concerning hom- 

 ogeneous immersion objectives. His 

 inquiries were, for the most part, met 

 in an article published in the same 

 number ; but since the information 

 desired is, perhaps, frequently asked 

 for by persons w^ho wish to purchase 

 objectives, we will attempt to sum up 

 the facts in this place. First, the 

 writer refers to the cost of objectives 

 of this class. It is true they are rather 

 high-priced, but it should be consid- 

 ered that they are designed to accom- 

 plish certain kinds of work, and they 

 should not, therefore, be compared in 

 cost with other lenses that will not do 

 the same work. The Hartnack 

 water-immersion, at $45, probably 

 w^ill not do what the Spencer homo- 

 geneous immersion, at $55, will do, 

 for the simple reason that its numer- 

 ical aperture is smaller. Nevertheless, 

 the Hartnack may be, and pi'obably 

 would be in this case, the better ob- 

 jective for the writer. 



The whole matter, in a nyt-shell, 

 is this : A homogeneous immersion 

 objective, of high numerical aperture, 

 if well made, is an invaluable aid to 

 microscopical investigations of cer- 

 tain kinds which require great resolv- 

 ing power for fine, close markings. 

 The use of such objectives is veiy re- 

 stricted, and, as a rule, they are not re- 

 quired in histological work. 



For a power as high as a ^ or a y^ 

 a well-constructed water-immersion 

 lens (which may be adjusted to work 

 also in glycerin if desired) will be 

 found the most generally useful in the 

 ordinary work of histology and com- 

 mon observation. Even for those re- 

 fined studies of bacteria and disease- 

 germs, which have attained such 

 great importance at the present day, 

 these objectives are to be most highly 

 recommended. The impression that 

 only oil-immersion objectives are suit- 

 able for such investigations is founded 

 upon false notions, and failure to dis- 

 tinguish ' between the conditions of 

 resolution of close markings and the 



definition of minute, isolated particles 

 or lines. 



Postal Club Boxes. — The boxes 

 were started on the circuits on the 

 iSth of September, and may be ex- 

 pected to follow each other at the rate 

 of two each month, every third box 

 being one of Cole's series. We have 

 received the following boxes : 



Box C E. This is one of Cole's se- 

 ries, containing a transverse section 

 of the stem of Equisetum arvetise 

 and of the root of l^araxacum ojffici- 

 nate^ with the usual descriptive text. 



Box F 2 contains some excellent 

 preparations, 



1. Orbitolites. F. M. Hamlin. 

 These shells, of a complex type of 

 foraminiferal growth, show ' the vari- 

 ous stages of growth and develop- 

 ment from nautiloid or spiral to cyc- 

 lical form.' The slide is described 

 as a new one for opaque objects. It 

 presents a very neat appearance, is 

 light and strong. It is made of a 

 piece of soft pasteboard. A half-inch 

 hole is made in the centre with a 

 gun-punch, the edges of the slide 

 bound with colored tissue paper, a 

 3 XI paper label, such as the opticians 

 have for covering slides, is pasted on 

 the back, and a colored piece of pa- 

 per for the bottom of the cell is fitted 

 in. A brass curtain-ring is then fixed 

 in the hole and the covei'-class is at- 

 tached to it. Then the face label is 

 put on and the work is done. 



2. Endothelium. S. H. Gage. 

 Stained with silver and carmine. 



3 . Developing germ of a tooth from 

 a human embryo. A. M. Ross. 

 Coupled with the careful description, 

 this is a very interesting specimen. 

 It is such preparations that are most 

 valuable to the club, as they give in- 

 formation. 



4. Proboscis, or ' tongue ' of moth. 

 John D. White. This slide is one of 

 Mr. D. Folsom's pi-eparations, which 

 is a good indication of its excellence. 

 Mr. White intends to replace it by 

 another slide, which was not ready 

 when his contribution was called for. 



