WITH THE MICROSCOPE. 263 



bone, dip it into the old collodion, pass it quickly and well over every 

 part of this surface of the plate. With the same cloth that polished 

 the back, rub this off briskly, then with the other clean perfectly dry 

 cloth finish off the polishing, so that when breathed on, the surface 

 may present a uniform dull appearance without any streaks ; set it 

 face down on a clean sheet of foolscap paper, or in a grooved well- 

 closed plate box, the finished faces all looking one way. Thus pre- 

 pare the number of plates required for immediate use. If to be kept 

 a few hours, wrap them up in another fold of paper, place them in a 

 dry drawer, always noting which is the perfectly cleaned surface. If of 

 a larger size than 6 inches square, it will be more convenient to clean 

 them on a proper polishing board. Cleanliness in this, as in the suc- 

 ceeding stages, is absolutely requisite. If no old collodion be at 

 hand, a polishing liquid may be made by- 

 Howard's precipitated magnesia, 20 grains. 

 Strong liquor of ammonia, i drachm. 

 Alcohol, 2 ounces. 



This, however, requires to be most carefully removed, in the clean- 

 ing, from the edges of the plates, or they would soon render the bath 

 alkaline. Or the method adopted by M. Boetter may be adopted for 

 cleaning chemical glasses which is strongly recommended by Mr. Carey 



Lea : 



Common sulphuric ax:id, i oz. 



Bichromate of potash, i oz. 



Water, i pint. 



The glasses are to be left in this solution for seven or eight hours, 

 their surfaces being entirely covered by it. They are then to be 

 rinsed well beneath a tap. The same solution answers many times. 

 If the plates are handled, it is as well to see there are no cuts or 

 abrasions on the fingers, for this fluid to come in contact with, 



342 Arranging the Camera. Supposing the portable form of 

 apparatus recommended by Dr. Maddox be selected, we proceed as 

 follows : A room is to be chosen which has a window with a south- 

 west aspect, or at least one where the sun's rays enter during the 

 greater part of the day. The end of the apparatus is placed outside the 

 opened window in such a manner that the face of the prism is directed 

 at right angles to the incident rays ; the legs of the triangle are set 

 apart so that the whole stands firmly on the floor. The object being 

 fixed, it is first carefully examined under the compound microscope, 

 and if of any depth, the part in strict focus when the best general 

 character of the object is attained, is well noted. The objective like- 

 wise being selected, is to be screwed into the neck of the microscope, 



