16 



On a New Low-Power Object Glass for the Microscope, 



By John Bockett. 



{Read Nov. 22nd, 1867.) 



It is only recently that our first-class opticians, after surmounting 

 tlie almost insuperable difficulties attending the manufacture of 

 the highest powers in connection with the microscope, have, as it 

 were, now turned their attention to the construction of compound 

 glasses of low angular aperture, but still possessing the great 

 desideratum, absolute sharj)ness. It is very desirable to possess 

 lenses capable of resolving certain objects into sti'ise and dots ; but 

 with due regard to the skill of the manufacturer, and the ability 

 of the manipulator of the microscope, the bulk of real work 

 is always done with medium and low powers. The plan 

 adopted in Germany, as seen in their wonderfully illus- 

 trated books on Botany, &c., deservedly ought to be followed : 

 they delineate the object as absolutely seen, and then pro- 

 ceed to show its structure and parts, as magnified with 

 various powers. Let us, therefore, begin with the whole, and 

 then analyse it with our high powers. That all who use the in- 

 strument will assert that this is the plan in vogue, I am prepared to 

 admit ; but it is to be feared that in the first place we begin our 

 story in the middle, and not at the beginning, and, in fact, with 

 good excuse : inasmuch, as a complete set of good objectives costs 

 a veiy large sum of money. Hitherto the range of low powers for 

 the microscope has been somewhat limited, the 2-inch, as a rule, in 

 comiection with 1-inch and two-thirds of an inch, being the general 

 standard; the 3-inch is of modem date, but has been found to be 

 an invaluable glass. Although the 3 -inch, with the first eye-piece, 

 magnifies only 12 or 13 diameters, it still became necessary to have 

 an objective even of lower power than this — say, of 8 or 9 diameters 

 — so that the whole of a beetle, flower, shell, or large section of 

 wood, could be seen in its entirety. This desideratum has just 

 been produced by Mr. Ross, of Wigmore Street, Cavendish Square, 

 and is termed by that eminent optician a 4-inch object glass. It 

 may be argued by some that a lens of such low power is really of 

 veiy little use, and, in point of fact, very little needed, seeing that 

 a hand-glass, or pocket magnifier, will enable a general view to be 

 obtained of all moderately sized objects. 



