670 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
(4) By inserting between eye and body-tube a Ramsden eye-piece, 
a single clear image of the signal, which can be adjusted on the cross 
wires, is obtained. 
(5) The above methods are equally applicable to the simple gonio- 
meter without telescope and collimator. 
C2) Eye-pieces and Objectives. 
Fluorite in Apochromatic Objectives.* — Mr. E. M. Nelson re- 
marks — “ As fluorite is becoming scarce, an important question arises 
as to whether fluorite is or is not present in any given lens. This can 
readily be determined by means of a polariscope. 
The Nicols are crossed, a 2-in. objective is placed on the nose-piece, 
and a low eye-piece employed. The various portions of the objective to 
be tested are unscrewed, and each combination is separately placed on 
a glass slip on the stage and examined in the dark polarized field. If 
the combination contains fluorite there will be a luminous white silky 
appearance, but if there is no fluorite, then the field will remain dark. 
The following are examples : — The Zeiss 24-mm. apochromatic has 
three elements, of which the middle contains fluorite. The apochromatic 
12 mm. has four elements, of which the second and back contain fluorite. 
The apochromatic 6 mm. has three elements, of which the middle and 
back contain fluorite. The apochromatic 3 mm. has five elements, 
and the last but one contains fluorite. 
But with regard to this last example, it should be noted that all 
3- ancji 2-mm. objectives are not alike.” 
C3) Illuminating- and other Apparatus. 
A New Spherometer. — Mr. E. M. Nelson communicates the follow- 
ing : — Nearly all spherometers are graduated in dioptries, as they are 
chiefly employed for measuring the foci of spectacles. This graduation 
is inconvenient if you require the radius only. The size of the sphero- 
meter ought to bear some proportion to the size of the lens to be 
measured, consequently for general purposes more than one spherometer 
would be required. A large lens, for instance, with shallow curves, can 
be more accurately measured by a large spherometer, which would not 
measure a small lens at all ; moreover, as spherometers are expensive 
instruments, and as at least three of them would be required, it seemed 
better to design one that would be suitable, and, at the same time, be as 
simple in construction and as accurate as any hitherto made. 
The usual form of spherometer is that shown on p. 131, fig. 19, of 
this Journal for last February, in the admirable paper by Prof. 
Silvanus Thompson on his new focometer. 
The principle on which these instruments depe nd is the measurement 
of the versed sine with a given chord. 
Now the defect in the principle employed in the spherometer shown 
in fig. 20 is that you cannot be sure that the chord between the two fixed 
points is that of a great circle of the sphere ; if it is not, the result will be 
inaccurate. 
Journ. Quek. Micr. Club, v. (1892) p. 122. 
