890 EECORD OF CUREENT RESEARCHES RELATING TO 



the stratum of air, using a pencil incident at the upper internal 

 surface just within the critical angle of emergence — the prismatic 

 rays of different refrangibility being then available. Objects in fluid 

 may be placed on the plane surface of the semi-cylinder and illu- 

 minated with ordinary transmitted light, or rendered " self-luminous " 

 in a dark field, as with the hemisijherical illuminator, prism, or 

 Wenham's immersion paraboloid. A concave mirror with double 

 arm is sufficient to direct the illumination. The semi-cylinder 

 figured was made in 1875 by Mr. Tolles, of Boston, for measuring 

 apertures. The mounting was exhibited at the Society in 1878. 



The Iris Diaphragm an Old Invention.* — It is generally st:p- 

 posed that the iris diaphragm, as applied to Microscopes and tele- 

 scopes, is a very recent invention, but the following passage, taken 

 from an early volume of ' Nicholson's Journal ' (1804), shows that it 

 is three-quarters of a century old : — 



" Every attentive observer must have taken notice, that light is of 

 as much consequence to artificial vision as magnifying power. It may 

 therefore afford matter of surprise that the most variable of all adjust- 

 ments of the eye, viz. that of aperture, should never be introduced 

 into our artificial combinations. Distant woods, and other land 

 objects, are invisible to a high magnifying power, for want of light, 

 when the same objects may be distinctly seen with a lower. By means 

 of an artificial iris, which an ingenious artist will find little difficulty 

 in contriving, this disadvantage in telescopes might be obviated. Sup- 

 pose a brass ring to surround the object end of the telescope, and 

 upon this let eight or more triangular slips of brass be fixed so as to 

 revolve on equidistant pins passing through each triangle near one of 

 its corners. If the triangles be sliddeu in upon each other, it may 

 readily be apprehended that they will close the aperture ; and if they 

 be all made to revolve or slide backwards alike, it is clear that their 

 edge will leave an octagonal aperture greater or less, according to 

 circumstances. The equable motion of all the triangles may be pro- 

 duced either by pinions and one toothed wheel, or by what is called 

 snail-work." 



Microscopical Goniometer.! — Mr. Eutlcy, referring to the 

 Schmidt goniometer (a positive eye-piece in which a cobweb is placed 

 with a graduated brass circle and vernier), says that when the angles 

 of crystals occurring in sections of rock which are not very trans- 

 lucent have to be measured by this instrument, difficulty is often 

 experienced in seeing the cobweb distinctly, and this is one of the 

 most serious drawbacks to the use of this kind of goniometer for 

 petrological purjioses. Its utility would, he considers, be increased 

 if one-half of the field were obscured by the insertion of a blackened 

 semicircle of metal v\ithin the focus of the eye-piece instead of the 

 cobweb. 



Pleurosigma angulatum as a Test Object. — We continually find 

 suggestions made in English and foreign journals that P. angulatum 



* ' Am. Journ. Micr.,' v. (1880) p. 136. 



t ' Study of Kocks ' (8vo, London, 1879) p. 53. 



