1896.J MICROSCOPICAL JOURNAL. 117 



will give a negative of proper contrast for priutiuy i)iir- 

 poses. This is the case with objects having but little con- 

 trast between their different parts, or those colored objects 

 in which the coloring is so faint that they fail to absorb 

 a sufficient number of the iin})iuging rays and cdnse- 

 qu(Mitly transmit so many that there is nearly as much 

 effect produced on the plate by the rays that pass through 

 them as by those which pass by them. 



To photograph such objects, it is necessary to make a 

 short exposure and to stop the development as soon as the 

 details appear and before any trace of a reduction of the 

 silver compounds appear in those places which should ap- 

 pear clear in the negative and, after fixing and washing, 

 to intensify the negative so that sufficient contrast may 

 be had for printing. Of the various intensifying methods 

 that by bichloride of mercury and aqua ammonite is the 

 best. The fixed and thoroughly washed plate is placed 

 in an aqueous saturated solution of bichloride of mer- 

 cury until sufiicient density is obtained, then thoroughly 

 washed to remove every trace of bichloride, after which 

 it is placed in dilute aqua ammonite to blacken, and again 

 thoroughly washed. 



While in the bichloride, density is best judged by view- 

 ing the plate by transmitted light, remembering that the 

 plates will be somewhat denser after passing through the 

 ammoniaj solution. The strength of the ammonite solu- 

 tion does not matter materially. Where the action of the 

 bichloride has been prcdonged, it is necessary to use very 

 strong aqua ammonia3 to blacken the plate entirely. 



In some cases reduction of a too dense negative may 

 be required, or it may be necessary to reduce a negative 

 in order to clear it before intensification. This is best 

 done by placing the plate in a solution of sodium hypo- 

 sulphite of ordinary strength to which a few grains of 

 potassium ferricyanide have been recently added. The 

 rapidity of the reduction depends upon the amount of ferri- 



