432 SUMMARY OF CURRENT RESEARCHES RELATING TO 



MICROSCOPY. 



o. Instruments, Accessories, &c.* 

 (1) Stands. 



Dick and Swift's Patent Petrological Microscope.f — Mr. A. Dick 



describes this Microscope (fig. 57) as follows: — 



" Many years ago I requested Mr. Swift to make for me a first-rate 

 Binocular Petrological Microscope. The centering of the stage by 

 screws was, I suppose, as good as it could be made. I found it unsatis- 

 factory when using high powers on small crystals. A centering nose- 

 piece answered no better. Only by the simultaneous rotation of the 

 polarizer and analyser by hand, little by little, could I keep the inter- 

 ference figures of small crystals in the field of view, or feel certain that 

 the figures had not left it during rotation owing to the eccentricity of 

 the centering. By small crystals I mean crystals under 1/1000 in. in 

 diameter, and of such thickness as one finds them at the edges of petro- 

 logical sections. Results obtained thereby were only slowly got, and 

 always with some uncertainty. I tried the Nachet Microscope, but 

 found it a cumbrous instrument. Latterly, I connected the polarizer 

 and eye-piece analyser by a jointed rod, and got thereby excellent 

 results, whilst I could still retain binocular vision for all but certain 

 observations. 



I suggested to Mr. Swift that he should manufactures more perfect 

 Student's Microscope than any now obtainable ; one which would suit 

 alike the mineralogical, petrological, botanical, or medical student. 

 Having agreed upon the design of the instrument, I left to Mr. Swift 

 the carrying out of the details, which he did in an ingenious manner 

 and with excellent workmanship. When the Microscope was finished 

 I went over it carefully, and handed it to several friends interested in 

 such matters for suggestions, all of which have been carried out. You 

 see the result in a small Microscope where there is little lumber and 

 much capability of good work. Its interest to the Mineralogical Society 

 lies in its adaptation to the study of the optical properties of minerals 

 generally, and particularly to that of the thin plates of minerals seen in 

 ordinary sections of rocks prepared for microscopical examination. For 

 this purpose the analyser and polarizer are connected together by toothed 

 wheels. They can thus be turned together in any position relatively to 

 one another — crossed, parallel, or inclined— each nicol being so fitted 

 that it can be set in any position. The wheels can be clamped in any 

 position. The tube of the Microscope is of the ordinary construction. 

 Within the lower part of it is a sliding tube w^hich carries a sliding 

 plate. In the plate are three circular openings, of which the central one 

 is always open. In one of the other openings is fitted a Klein's plate ; 

 in the other a lens. The lens can be easily removed and another of 

 different focus put in its place, according to the purpose for which it is 

 to be used. 



The lens of shorter focus brings interference figures into the eye- 

 piece, where the dispersion may be studied, and also where the apparent 

 angle in air of a biaxial crystal may be approximately measured, if the 



* This subdi-vision contains (1) Stands; (2) Eye-pieces and Objectives; (3) Illu- 

 minating and other Apparatus ; (4) Photomicrography ; (5) Microscopical Optics 

 and Manipulation ; (6) Miscellaneous. 



t Mineralogical Magazine, viii. (1889) pp. 160-3. 



