lOB SUMMARY OF CURRENT RESEARCHES RELATIKG TO 



Judging from past experience, it will probably be too much to 

 expect that the desired change should take place all at once, and 

 that the A, B, C, &c., or Nos. 1, 2, 3, &c., should forthwith be swept 

 away, but we would venture to suggest that the power of the eye-piece 

 should be indicated in the catalogues and elsewhere, as well as the old 

 title, and if this were done we are sm-e that the latter would soon be 

 wholly disused. 



The tables of magnifying powers issued by opticians are at 

 present, in many cases, of a very misleading character, not so much 

 from the fact that the objectives are underrated — a true -j^o-inch being 

 called a i-inch — but that, according to the tables, one and the same 

 eye-piece magnifies differently when it is used with different objec- 

 tives ! 



We have accordingly compiled the annexed table of magnifying 

 powers for ready reference. It includes all the more usual objectives, 

 and the full series of eye-pieces of Messrs. Beck, Powell, and Boss. 

 It will be noticed that the magnifying powers of the No. 1 or A 

 agree in all three cases, those of the No. 2 or B slightly varying, 

 being 8, 7*4, and 8. It would be an improvement if they could 

 all be made 7^, which would preserve the uniformity of the series. 

 The No. 3 or C vary greatly, being 15, 10, and 12^. The No. 4 or 

 D agree, whilst No. 5 or E are 25, 30, and 25. 



We think that an ideal series should run thus : — No. 1 = 5, 

 No. 2 = 7i, No. 3 = 12^, No. 4 = 20, No. 5 = 30. 



With the exception of the -^3, ^V? ^^^ tV? ^^^ *^® objectives 

 included in the table are actually constructed by English or foreign 

 opticians. As objectives are, however, not uncommonly found to 

 vary somewhat from the designated focal lengths, the figures for the 

 ^, -^j, and -^^ have been retained. 



The length of tube is assumed as usual to be 10 inches. 



Braham's Microgoniometer.* — At a recent meeting of the Bath 

 Microscopical Society, Mr. Braham described a microgoniometer for 

 measuring the angles of crystals. " The body of the microscope-tube 

 is formed at right angles. A rectangular prism is so adjusted that 

 the plane of the hypothenuse is at an angle of 45 degrees to the axis 

 of rotation. On bringing any crystal into the centre of the field, a 

 fibre in the focus of the eye-piece is made to coincide with either of 

 its edges so that the degrees passed through can easily be read. 

 Thus, as the instrument measures a magnified image of the crystal, 

 and the object itself is stationary, it will readily be seen that the 

 angles of any crystal visible under the highest powers of the Micro- 

 scope can easily be measured." 



Watson's Sliding-box Nose-piece. — Messrs. Watson have recently 

 contrived a sliding-box nose-piece to carry (1) the vertical illuminator 

 (Fig. 16), or (2) the analyzing prism (Fig. 17) of the polarizing 

 apparatus, or (3) the binocular prism. The application of an extra 

 nose-piece in this form appears to be convenient. Experience must, 



* Engl. Mech., xxxiv. (1881) p. 277. 



