lioijal Microscopical Society. ' 253 



groove in a block of wood of such a size as to make the central 

 point of the negative coincide with the central point of the lens ; 

 the frame for the negative is kept in place by a spring, and the 

 block can be screwed down at any point of the slit before men- 

 tioned. The space between the negative and front of the camera 

 should be covered with a focussing cloth, so that no light should 

 enter the lens except through the negative. To prevent the print 

 being reversed, it is necessary to take the impression through the 

 back of the plate. The apparatus being properly arranged, the 

 whole is turned at an angle towards the sky so as to be clear of 

 trees or other obstructions near the horizon ; direct sunhght is not 

 required, and indeed is detrimental to this part of the process, 

 although Moitessier recommends a complicated system of con- 

 densers ; but these are superfluous when the enlargement required 

 is so small. There are several precautions to be used in preparing 

 the plate to receive the image ; in the first place, the collodion must 

 not be too thick, for, if so, it has two disadvantages, — the whites of 

 the image are sure to have a yellowish tinge, and the film is apt to 

 slip ofi" either in the nitrate bath or during the subsequent opera- 

 tions, so that it is better to add a small quantity of ether (3 j to 5 j). 

 In the next place, previously to pouring on the collodion, the plate 

 must be rubbed over by means of a bit of rag, with wax dissolved 

 in ether; care must be taken not to apply too much, for in that 

 case it forms reticulated markings on the film ; nor too little, or 

 else the collodion will not come off the glass in the succeeding parts 

 of the operation. The glass plate must be coated as thickly as 

 possible with the thinned collodion, as it will then come ofi" more 

 easily. Having taken the image of the negative and developed it 

 in the usual manner (I find the gelatino iron developer answers 

 extremely well for this process), the next step is the toning; this 

 is best accomplished by means of chloride of gold, which gives a 

 good black ; platinum is, I think, not quite so good ; other sub- 

 stances may be used, but they do not answer so well ; uranium 

 gives an ugly reddish- brown colour ; bichloride of mercury, with 

 the subsequent addition of very weak solution of hyposulphite of 

 soda (gr. 1 to 5 j water), gives a good colour, but is excessively 

 troublesome to use, as the mercury makes the film very rotten ; so 

 gold, although expensive, is the best ; gr. 1 to 5 j of water is poured 

 over the collodion positive until the black colour is seen through 

 the back of the plate when held over a dark material, such as 

 velvet. It does not do to hold it up to the hght, for then 

 the print may look toned when it is not so. When the above 

 quantity will not tone any longer some more gold must be added, 

 but the remainder need not be thrown away, as it keeps well 

 and will do again another time. The efiect of this procesd 

 may be varied according to the subject ; if the plate is exposes 



