264 On a Proposed Standard Micrometer Eye-piece. 
I have yet to gain the opportunity of observing them in life ; and 
though, as I suspect, the species may be numerous and examples 
abundant, minuteness to T yo) and parasitism will oppose so 
many obstacles to the inquiry, that by vigilant and patient research 
alone may we hope to gain any further acquaintance with them. 
YI . — On a Proposed Standard Micrometer Eye-piece, or Ei do- 
meter ; and on a uniform and easily applied Method of Naming 
Microscopic Objectives. By George Findley. 
With a proper micrometer eye-piece, nothing can be easier than to 
see the exact magnifying power of any microscopic objective * at any 
interval of object and image, and of naming it accordingly. 
I have had one of my own adapting in use for over six months, 
/* X ^ 
calculated according to the usual formula for eye -pieces, ; 
where / is the focal length of the field-lens ; e, that of the eye-lens ; 
and cl is the distance between them. I think it would help to settle 
some of the disputes among microscopists as to the solar focal length 
of objectives. Some of the uses it may be put to are the following : — 
1. Place on the stage of the microscope a micrometer ruled to 
xYo-ths and T oVoths of an inch, and the eye-piece shows at sight how 
many xwoths or yoVoths in the eye-piece micrometer are covered by 
the image of iYoth or roVoth on the stage micrometer. The size 
of the object ywoth or T oWth is known ; the objective makes a 
magnified image of it, which is measured by the eye-piece micro- 
meter. The ratio of the size of the seen and measured magnified 
image, to the size of the known unmagnified object, gives the mag- 
nifying power of the objective. If there be a draw-tube, there will be 
shown the increase of size given by the increase of length of tube. 
2. My eye-piece of itself magnifies the image just 10 times, 
making the total magnifying power with it, just 10 times that of 
the objective alone. 
3. It furnishes us with the means of knowing the magnifying 
power of any other eye-piece. 
4. It enables us to tell the real size of the original object. 
5. It enables us to calculate the principal focus of the objective, 
from the seen magnifying power and the interval of object and 
image, i. e. the distance of the conjugate foci from each other. 
6. Its power to magnify 10 times may be shown to the eye. 
7. It renders practicable the method of naming objectives ac- 
cording to their seen magnifying power at a standard distance, say 
* Simple or compound, dry or wet, object covered or uncovered. 
