232 
SUMMARY OF CURRENT RESEARCHES RELATING TO 
aperture. The position of the lever may now be called 6 J ; if that is still 
not enough we may turn the lever a third 60°, which will bring it home 
to the right hand, its full movement being a half circle ; the position 
may be called 6|. The next step is to turn the lever back to the left, 
and turn on the No. 7 diaphragm, and so on until the full aperture of 
the condenser is employed, when the lever must be placed to the left 
Fig. 34. 
hand to correct the aberration for the extreme aperture. Thus we can 
by means of the correction collar graduate the aperture with the same 
facility as with an iris, and we can record any particular aperture with 
a degree of accuracy foreign to the iris. 
It should be mentioned that it is, of course, necessary to refocus the 
condenser after moving the collar, just as with an objective.” 
New Magnifying Lens with Combined Illuminator. — At the 
Society’s meeting on March 20th last, Mr. E. M. Nelson read the 
following note : — “ This magnifying lens or loup is the outcome of a 
paper I read before this Society on lens mirrors the year before last.* 
It consists of a lens mirror with the silver removed in the centre, and 
another lens placed above it (fig. 35). The focus of the lens mirror is 
the same as that of the combined lenses, so that the light reflected by 
the lens mirror is focused on the object when the object itself is in focus. 
Fig. 35. 
This is the first Messrs. Watson Lave made from my design ; it is not 
achromatic, but the curves have been calculated for minimum aberration 
for the system. The aberration is half that of an equi-eonvex lens of 
the same power, viz. 8. The curve of the lens-mirror being fixed, it is 
not possible with this simple form to reduce the aberration lower. The 
result is very good and the definition is clear and bright. It is in fact 
the combination of a loup and a lieberkuhn. The lens mirror not only 
gives more light, but is cheaper than a speculum, it also forms one of the 
lenses of the combination. 
This Journal, 1894, p. 254. 
