Monthly Microscopical 
Journal, Sept. 1, 1870. 
of Microscopic Objectives. 153 
apparatus, and at an early period of the investigation ; the follow- 
ing are by the second method alone, and were taken after consider- 
able practice in using the apparatus. 
Hartnack, No. 9. Tolles, 4. Natchet, No. 7. 
Adjusted for Adjusted for Adjusted for 
Object Covered. |Object Uncovered.} Object Covered. |Object Uncovered.} Object Covered. |Object Uncovered. 
¢= 248 mm, | /= 249mm. | 7/= 257mm. | /= 258mm. | 7= 265mm, | 7 = 265 mm. 
; N. iF N. if N. Mie N. if N. ie N. 
‘0819 10 | :0917 10 | :0640 10 | :090 15 | -0721 10 | ‘0898 15 
‘0816 10 | ‘0911 10 | :0636 10 | -0898 15 | °0721 10 | -0898 15 
‘0816 10 | :0910 10 | :0636 10 | ‘0898 15 | :0718 10 | -0897 10 
M.=:0817 | :0910 15 M.=:0638 M.=:0899 | :0718 10 | :0896 15 
P. E.=:00006 M.=-:0912 | P. E.=00001) P. E.= "00004; M.=:0719 M.=:0897 
p. e-="00010) P. E.=:00009) p.e.="00002) p.e.=-00007; P. E.=:00005) P. E.=-00003 
p. e.='00019 p.e.="00010) p, e.=:00006 
1? = 249 mm. 
ifs n. | 7= 264 mm. 
‘0817 10 ‘fc N. 
‘0817 10 | :09238 10 
0817 10 | 0914 10 
‘0817 10 | 0911 10 
‘0816 10 M.=:0915 
M.=:0817 | P. E.=:00020 
P. E.=-00002| p. e.=:00034 
p. e.='00006 
In the two preceding Tables the focus f is given in decimals 
of an inch, as computed from each reading. N is the number of 
spaces observed on the micrometer scale, and 7 the distance between 
the two scales. The most probable mean m is computed by giving 
weight to each observation proportional to the number of spaces Nn 
taken; that is, assuming that the whole error lies in reading the 
size of the magnified image on the paper scale. p. E. is the pro- 
bable error of the mean m, and ». e. that of one observation. 
In all the measurements the objective was refocussed for each 
reading, and in those given above it was dismounted and remounted 
for each set. 
The Table below gives the result of several hundred measure- 
ments on various objectives. The first column gives the order in 
the Table for convenience of reference ; the second the name of the 
maker and designation as given by him; the column headed a gives 
the adjustment, whether for covered or uncovered glass, with the 
position of the index, if this is present; also whether the lens is 
wet or dry if an immersion lens. The next column gives in milli- 
metres the distance denoted in the formula by 7; that is, the dis- 
tance between the two scales. ‘The value of f is in all cases the 
mM 2 
