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III . — Notes on the Use of Mr. Wenhcinis llejiex Illuminator. 
By Henry J. Slack, F.G.S., Sec. RM.S. 
{Read before the Royal Microscopical Society, June 2, 1875.) 
If Mr. Wenham’s Reflex Illuminator for High Powers is used 
under the circumstances for which he especially contrived it, little 
difficulty will be found with , suitable objects. The light, as he 
explained, penetrates only where the object makes a new surface on 
the slide, and “ acts,” to use one of his familiar phrases, “ like a hole 
in a dark lantern.” 
The effect is so admirable upon many objects, such as scales of 
insects, certain micro-fungi, minute algas, desmids, diatoms, &c., 
that everyone who has successfully tried it must wish to add to its 
range of utility, and this may be easily done. 
It will be found that most balsamed objects, and many in which 
the covering glass lies very close to the slide, give with it so much 
false light when ordinary objectives are employed, that the result is 
very unsatisfactory. This false light will be found in many cases so 
oblique that it can be got rid of by using an objective with a small 
angle, or temporarily reducing the angle of an ordinary high power 
by a movable stop. 
For example, a slide of Surirella gemma and this illuminator 
exhibited no false light with a glass of about 7 0° ; some, but not 
much,' with a fine fill made on Mr. Wenham’s new formula, and 
having an angle of 150°, too much to be endurable with Powell 
and Lealand’s immersion f th full aperture ; and none with the 
same glass and with a stop limiting the rays admitted to about 90°. 
Many slides of butterfly and other scales taken at random from a 
cabinet become manageable with reduced apertuies, and the effects, 
when the plan succeeds, are very curious, beautiful and instructive. 
Mr. Wenham has alluded to the changed aspects obtained by 
rotating the apparatus when employed upon the so-called Podura 
scale, Lepidocyrtus curvicollis, and similar observations may be 
made with regard to Lepisma scales, and those of various insects 
allied to Podura. Indeed it is not prudent to pronounce an opinion 
upon any scale of difficulty until this method has been tried, and 
all the aspects it produces considered in their mutual relations. 
It is by no means intended to advise microscopists against the use 
of this apparatus with large-angled glasses upon objects mounted 
so as to be fit for it ; but when slides fail, the observer is recom- 
mended not to abandon the plan, but to reduce the angle of the 
glass and try again, and with good chances of success. The appa- 
ratus has a remarkable power of increasing both the penetration 
and the resolution of good objectives. 
