870 



RECORD OF CURRENT RESEARCHES RELATING TO 



Fig. 84. 



Holmes's "Isophotal" Binocular Microscope. — The description 

 of this Microscope should be added to the " Curiosities of Literature." 

 The paper was recently presented to this Society, but withdrawn on our 

 objection to print it au serieux and without alteration, having appa- 

 rently met with a similar objection at the hands of the British Asso- 

 ciation authorities last year. Deviation is described as being " half 

 as great when an isosceles prism is used," it being apparently supposed 

 that an " isosceles " prism has an invariable angle instead of an 

 infinite variety of angles. A prism is commended as giving " flatness 

 of field." Reflecting prisms are described as causing more loss of 

 light and more error than refracting ones, and notwithstanding a 

 5 per cent, loss of light by transmission, a net gain of 5 per cent, is 

 vouched for as the result of the interposition between the objective 

 and the eye-piece of the refracting "Isophotal" prism (!). The 

 " distance of the prism from the objective in relation to the eye-piece " 

 is held to have an efiect on the angles of incidence and emergence. 

 Wenham's prism is denounced as causing 

 the " left-eye view to be darkened, definition 

 impaired, field cylindrically distorted," and 

 Stephenson's as having prisms with " twelve 

 surfaces" (instead of six or eight), and as 

 "practically useless from the line of sight 

 being at right angles to the objective, and 

 from torsion of the image." An achromatic 

 prism is italicized as having only "three sur- 

 faces " (instead of four), and finally a right- 

 angled prism is apparently in future to be 

 known as a " Holmes prism." 



The Isophotal prism as drawn by the 

 inventor is shown in Fig. 84 (fac-simile). 



The following is the inventor's descrip- 

 tion verbatim:* — 



" In the course of experiments to produce 

 a Microscope that might be used with both 

 eyes, I have originated three constructions. 

 The first of these acted by total reflection, the second by free trans- 

 mission through divided glasses, and the third by an achromatized 

 jprism." 



[The first f and second J plans are then described.] 

 " With regard to my third and last plan of binocular Microscope, 

 by means of an achromatic prism. The advantages I claim for it are 

 (1) that it can be used as a monocular or as a binocular without 

 change of body-tube ; (2) that it gives two equally lighted fields in 

 two equally inclined body-tubes ; (3) that it gives stereoscopic effect 

 with less loss of light or definition than any other construction with 

 undivided glasses; and (4) from its evident adaptability to higher 

 powers." 



* 'Engl. Mech.,' xxxi. (1880) p. 464. 



+ ' Journ. Quek. Micr. Club,' i. (1869) p. 175. 



J ' Mon. Micr. Jouru.,' iii. (1870) p. 273. 



