408 J. J. Woodward— Nomenclature of Achromatic 
Mr. Cross (loc. cit. p. 409) has already pointed out this cir- 
cumstance which results from the fact that the modern achro- 
matic objective has considerable thickness, from its anterior to 
its posterior surfaces, and that it has properly speaking no true 
optical center. He gives two examples, in one of which a 
change of 5-24 inches in distance corresponded to a change of 
‘0067 inch in calculated focal length; in the other a change of 
6°10 inches in distance corresponded to a change of -0029 inch 
in calculated focal length, the objectives used in the a 
ment being uncorrected ths, so called. If, however, Mr. 
had used for this purpose lower powers, or had made greater 
variations in the distances employed, he would have found much 
greater discrepancies. For example, by measuring the magni 
fying power first at 25 and then at 50 inches, and deducing the 
value of f by the formula of Mr. Cross, I obtained in two cases 
the following equivalent focal lengths. For a so-called 1; inch, 
at 50 inches distance, 1:2187 inches; at 25 inches distance, 
1-2468 inches; difference (0271 inch. For a so-called }th, at 50 
inches distance, 1982 inch; at 25 inches distance, ‘1893 inch; 
difference 0089 inch. 
Moreover, since the achromatic objectives of different makers 
are constructed on different series of curves, and the component 
lenses placed at different distances apart, it will be found that 
if two achromatic objectives magnify the same at any give 
sean, they will no longer do so if the distance is materially 
changed. : 
Hence I am compelled to agree fully with the observations 
of Mr. Cross (loc. cit. p. 401), that the nominal focal length 
signed to an achromatic objective can only serve in any case of 
‘‘a general appellation serving to group together bar's 
approximately the same magnifying power,” and must conclu 
therefore, that the English an 
ses no real claim to strict scientific accuracy, an Jestial 
that the comparison made by some with the case of the 
telescope is not vali 
{ 
p 
But besides the inevitable inaccuracy resulting from this . 
source, there are in the case of the higher powers of modern 
ers two other sources of much more considerable be 
The first of these involves the case of all those objectives 7” a 
are provided with a screw collar to correct for thick ‘wo 
cover; the second involves the case of objectives » but 008 
: 
: 
fronts, one for wet and the other for or those wit 8 
front which can be used wet or ea merely chang!g- rane 
correction given by the screw collar. i, well 
The correction for thickness of cover is made, aS bine 
combine 
known, by changing the distance between the front 
on of the triplet and the posterior two combinations As i 
