ithly Mi ical u 
ea daly © iil. of Immersion Lenses. 21 
At present, after much opposition in this country, the method 
(so much adopted abroad, in Europe and America) of immersion 
lenses has gained a remarkable footing, water being employed be- 
tween the objective and the covering glass. 
I propose in the following papers to give an account of some 
researches in this difficult subject, which I hope may tend to sti- 
mulate others to more successful exertions than my own. 
And to introduce the reader at once to the subject-matter, I will 
just mention here some results already obtained, which seem to 
repay the trouble of making the tables. 
The greatest aperture of a pencil divergent from a brilliant par- 
ticle immersed in Canada balsam and covered with a thin plate-glass 
cover, is only 
2x 41° 48’ of 93° 36’. 
When water is substituted for air this is enlarged to 
2 x 62°57’ or 125° 54’. 
If a flint-glass cover could be made, the divergent pencil ad- 
mitted to the object-glass when oil of turpentine is the immersion 
fluid would mount up to 
278° 37, or W572 147. 
Another point of most essential importance is the actual deviation 
of the pencils. The great desideratum is a minimum deviation for 
all pencils, and therefore a greater simplification of the refractions. 
On this point, to tempt the reader to attentively consider the sub- 
ject, I will venture to assure him that the quantity of light and the 
chromatic dispersion of the divergent pencils of the object is won- 
drously changed according to the optical nature of the film —be it in 
water or other fluid—interposed between the objective and the cover- 
ing glass, though the thickness of this delicate plate may not exceed 
half a hundredth of an inch. Thus it will be found that calling 
the quantity of light gained by a given dry or pneumo-objective 
unity, that for other arrangements it may be thus arranged :— 
Quantity of Light. 
Dry objective .. con. foes al 
Waterlensia. se, cite oes sssi, Cae 
Oil of turpentine 3 
Once more; I am about to discuss the amount of deviation for 
different media from 1° to 90° of incidence, occupying this minute 
yet interesting interval. That the deviation for moderate angles is 
three times less for the water lens than for the dry is a novel finding 
which should stimulate inquiry. It is probable that the means really 
at our disposal for improving even our best glasses are very far from 
being exhausted. Let not our motto be, “It is enough, rest and 
