NATURAL HISTORY MISCELLANY. 445 
Tue FOCAL LENGTH OF MICROSCOPIC OBJECTIVES. — Mr. C. R. Cross has 
ably discussed this subject in the ** Franklin Journal.” He remarks: ‘* The 
investigation of which the present article is a summary, was undertaken 
in order to see if some reliable method of measuring the focal length of 
mi iproscope objectives eue ot be found. The importance of such a 
method will be apparent to all who have had occasion to make use of 
objectives by different makers. The focal length of lenses of the same 
their relative oen For example, if two quarter-inch objectives 
be compared, and one Ee results much superior to that given by the 
other, we iaa pa at all sure that the better lens is not really of shorter 
focus than its designation would indicate." He presents a table giving 
** the results of several hundred measurements on various objectives, and 
suggests that an examination of the table will show that the focal length 
of the objectives of some makers differs considerably from the length 
marked upon them. For example, No. 34 marked 1-2 inch is really a 1-3 
inch objective; No. 33 marked 1-4 inch is really a 1-5 inch; No. 29 marked 
4-10 inch is really a 1-4. Lens No. 14, marked 1-4 inch, is really a 1-5 inch; 
but Nos. 18, 15, by the same Papi are correctly designated 1-5 inch, 
2-3 inch. Differences of this kind must of necessity lead to a great con- 
fusion in comparing objectives vith c one another. I would therefore 
suggest that each objective made should be measured before being offered 
tube, sliding in the tube of the microscope, and measure as I have already 
described. The draw-tube should be moved till the front of the ruled 
glass shall be exactly 10 inches from the micrometer used as the object. 
Or it would be more convenient still to have an apparatus similar to the 
first form, but arranged with a suitable stage and stand so that it can be 
Set at any desired angle. The distance 10 inches (254mm.), euggeste ed as 
s well as about the length used by microscopists in actual work. 
ANTHROPOLOGY. 
PERUVIAN ARCH.EOLOGY. — The extent to which the conditions of man- 
kind are influenced by natural circumstances, and how these may dictate, 
not alone the architecture and arts of a people, but their social, religious 
E opi organizations, is perhaps nowhere better illustrated than in 
The Inca Empire, it seems to me, was only rendered possible by 
RG ier geographical and topographical position occupied by the 
family or families that were its founders. Long antedating that empire 
its vast area contained a great number of communities, tribes, or princi- 
palities, more or less advanced or civilized, separated from each other, 
