40 REPORT— 1872. 



measured. Working in this way there is little difficulty in giving consecutive 

 plates any relative exposure that may be required. A collateral advantage is the 

 possibility of operating at any time of the day or night. 



With regard to the preparation of the plates, the author has latterly been using 

 the tannin process introduced by Major Russell. A preliniinarj^ coating with dilute 

 albumen is generally advisable, as any loosening of the film from the glass must be 

 avoided on account of the distortion that it might introduce. In some states of the 

 collodion an edging of black varnish put on after the exposure is sufficient to hold 

 the film down. The glasses, after being coated with collodion (Mawson's was 

 used), are immersed as usual in the silver bath, and then allowed to soak in disjilled 

 water, best contained in a dippiug-bath. They are then washed under a tap for 

 about half a minute, and put into the tannin solution (about 15 grains to the ounce) 

 held, in the author's practice, in a small dish. The author usually prepares his 

 plates in the evening, standing them up to dry on blotting-paper. In the morning 

 they are in a fit state for use. Artificial heat might no doubt be used if a more rapid 

 drying were desired. 



At a distance of 1 foot from the lamp tlie exposure required is four or five 

 minutes. The development is the most critical part of the process. The pyrogallic 

 solution should contain plenty of acid (acetic or citric), and its action must not be 

 pushed too far — the mistake wliich a photogTapher accustomed to negative work is 

 most likely to make. At this stage the spectra given by a candle-flame are not 

 very brilliant, on account of the iodide of silver still covering the parts which are to 

 be transparent. Any trace of fog is especially to be avoided. The author has expe- 

 rienced advantage in many cases from a solution of iodine in iodide of potassium 

 applied to the film previouslj' to fixing ; but its action must be carefully watched, or 

 too much silver will be converted. The iodide of silver is then cleared away with 

 hyposulphite of soda or cyanide, followed by a careful washing under the tap. 



With regard to the gelatine copies, the author has not much to add to the account 

 read before the Royal Bocietj'. The process is very simple and some of the results 

 very perfect, but he has not hitherto succeeded in sufficiently mastering the details. 

 Plates apparently treated in precisely the same manner turned out very diflerently. 

 That difficulties should arise is not yery extraordinaiy, considering the novelty of 

 the method ; but it is curious that some of the very first batch prepared are among 

 the best yet produced. The value of the results is so great, tliat the author has no 

 intention of abandoning his attempts, and perseverance must at last secm-e success. 



The author then said a few words about the performance and prospects of the 

 new copies. Their defining power on the fixed lines in the solar spectrum is all that 

 could be desired, being, so far as he can see, in no way inferior to the originals. In 

 the third spectrum the 3000 to the circle-gratings show the line between the D's, 

 if the otlier optical arraiu/emoits are suiiahlc. The fourth line of the group h is di- 

 stinguished with^the utmost ease. Theauthor isnot sufficiently familiar with spectro- 

 scopic worlc to make an (?xact comparison, but presumes that two prisms of C0° at 

 least would be required to eftect as much. The authorishere speaking of photogi'aphs 

 on worked glass. With ordinary patent plate, although veiy good results may be ob- 

 tained if tested by the naked eye onlj^, it is a great chance whether the magnifying- 

 power of a telescope will not reveal the imperfect character of the surface. 



With direct sunlight the light is abundantly sufficient ; but it is here in all 

 probability that the weak point of gratings lies, 'it should be distinctly understood 

 that where light is deficient gratings will not compete with prisms. There are 

 cases, however, where the scale might h& turned by the opacity of all highly dis- 

 persive substances to the rays under examination. Even if glass be retained as the 

 substratum, it may be used in a very thin layer, while prisms are essentially thick. 

 The immense advantage of a diffi-action-specitrum for the investigation of dark heat 

 need not here be insisted on. Taking all things into consideration, it is probable 

 that photographed gratings will supersede prisms for some purposes, though 

 certainly not for all. 



The specimens exhibited by Mr. Ladd are copies of two gi'atings by Nobert, 

 each of a square inch in surface, the one containing 3000 and the other 6000 lines. 

 The latter cost about £20. 



