APPENDIX. 345 



glass screen. The aperture in the shutter is then closed by means of 

 ths brass plate with handle inside the room, the sensitive plate sub- 

 stituted for the plate-glass screen in the plate-holder, and the expo- 

 sure made by opening and closing the moveable shutter by the means 

 already described. The time of the exposure is noted by the beats of 

 a metronome, adjusted to strike at second intervals, the dimness of the 

 yellow light in the room rendering the use of a watch inconvenient. 

 Having obtained the negative, a stage micrometer is substituted 

 for tha object photographed, and its divisions, as projected upon a 

 piece of ground glass held in the plate-holder, are carefully traced 

 upon paper. By comparing these with a standard scale, the exact 

 amplification of the object as represented in the negative, is readily 

 calculated. Other negatives, representing the same magnifying power, 

 can then be taken at any time by using the same objective and placing 

 the car at the same distance from the microscope. The ordinary 

 wet collodion process is the one used in the preparation of the nega- 

 tives." 



A bright white cloud illumination is obtained by throwing the 

 beams of light from the mirror on to a piece of greased ground glass 

 placed in the short body of the microscope, below the achromatic 

 condenser, by which the interference lines so often resulting from 

 employing the unmodified sun's rays are destroyed, and long expo- 

 sures with high powers permitted. In some cases Dr. Woodward 

 omits this ground glass. The objectives and amplifiers, as made by- 

 Mr. W. Wales, of Fort I^ee, New Jersey, " are specially corrected so 

 as to bring to one focus the rays in the violet end of the spectrum, 

 where the actinic power resides." 



The violet light is also " obtained practically pure by interposing 

 in the solar beam reflected from the mirror a shallow cell, with plate- 

 glass sides, containing a solution of ammonio-sulphate of copper." 



When other objectives have been used they have been the ordi- 

 nary achromatic lenses of other makers ; the i-5oth of Messrs. Powell 

 and Lealand gave excellent results in the hands of Dr. Curtis, as 

 exemplified by the prints sent to this country. Dr. Woodward 

 lately forwarded to Dr. Maddox some observations for publication 

 (which were placed in the hands of the editors of the Microscopical 

 Journal), on the result of comparative experiments made with a 

 flint-glass prism and lens to increase the dispersion of the violet ray 

 for the necessary exposure, and the ammonio-sulphate of copper 

 cell, the light employed being the same, in both cases reflected 

 from a plane mirror. It was found by using a sensitised collodion 

 plate that the actinic power was in favour of the light transmitted 

 through the cell ; or, in other words, that the loss by dispersion was 



