PROCEEDINGS OF THE SOCIETY. 



principal reason being of a two-fold nature [nasmucfa as the N.A. of 

 a photographic combination is ascertainable by dividing 0*5 by the 

 ratio, and as the/ ratio of lenses of this time did not exceed //8, the 

 power of picking oui details was consequently poor and ineffectual. 

 Moreover, as the construction of objectives of this period was not of 

 the same high order as that available in the present day — the astigmat 

 not being then known the whole of an insect, for example, could not 

 be rendered in focus at one and the same time. If the body was sharp, 

 the leei weiv fuzzy, or vice versd. The roundness of the field was very 

 marked and annoying. All these troubles were swept away at one 

 moment by the introduction of Zeiss's "Planar," for it has an 'excellent 

 flat field, and seeing its aperture is in the region of//?,- 5, its powers of 

 definition are exceptionally fine and beautiful. Even this system seems 

 id have been improved by some of the firm's later productions, where 

 three and even two-lens systems are employed instead of four, which 

 reduces the internal reflections from 28 to 15 and 6 respectively, a great 

 gain being thereby effected in making the "blacks" of the object' still 

 more intensely black. Other manufacturers have risen to the occasion 

 of later years ; there is the Micro-Summar, a lens by Leitz that is much 

 sought alter.- the Beck-Steinheil, by R. and J. Beck, which is a very 

 excellent tool and a two-lens system ; and lastly, a still later combina- 

 tion by Herr Reichert, called the " Polar," that is one of very exceeding 

 merit, furnishing images of great perfection at almost full aperture. 



In making slides for colouring purposes, it is imperative that the 

 background should be perfectly clear and clean, and that all the trans- 

 parent parts of the object should be absolutely transparent, free from 

 or veil of any kind. It is necessary also that the " blacks " should 

 really black, and that there should not be even a trace of brown in 

 lence, for if so, the truth of the colour rendering in the final result 

 would be imperilled almost for certain. An artist" friend of his and 

 hunseli had been for the past ten or more years engaged in trying to 

 conquer the many difficulties connected with this colouring of lantern 

 slides, and it would be well at once to state that she it was who had 

 originated and discovered the several means of producing the results 

 ibout to be shown. There were several difficulties, the principal being, 

 first, to get the painting clear and transparent, that there should be no 

 opaqueness m the slide, for if so, it would look dull and uninteresting 

 thrown on the screen, and would want such a powerful light to 

 rhen came the great difficulty of getting rid of the paint 

 anules, or " paini -rains " as they used to be called! Then the avoid- 

 e ol any running together, blending or overlapping of the colours at 

 '"i "f contact, which in some instances arose apparently from 

 ?ular absorption of the colour by the gelatin of the emulsion. Lastly, 

 .""' ddlieuln oi the paint hiding or rendering too faint the limiting 

 Kings in the specimen— as for example, the cell-walls of a piece of 

 or similar structure. This may seem of little importance, but, 

 '"i"''":' L "ledide lose, a very large portion both of its effectiveness 

 as ueii as its truthfulness. These difficulties, he thought, had at length 

 been overcome. s 



• fifty or more slides were shown. These comprised botanical 



