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 vv^ere,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 who wish to purchase 

 objectives, we vs^ill 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 

 will 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 probably 

 would be in this case, the better ob- 

 jective for the writer. 



The whole matter, in a nut-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 inarkings. 

 The use of such objectives is very re- 

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

 quired in histological work. 



For a power as high as a ^ or a Jg- 

 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 

 1 8th 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 Eqiiisetum arvense 

 and of the root of Taraxacum offici- 

 nate^ with the usual descriptive text. 



Box F 2 contains some excellent- 

 preparations, 



1. OrbitoUtes. 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 cover-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 preparations, which 

 is a good indication of its excellence. 

 Mr. White intends to replace it by 

 another slide, which was not ready 

 ^vhen his contribution was called for. 



