Motion Picture News (Sept-Oct 1916)

September 30, 1916 ACCESSORY NEWS SECTION 2079 Color Photography Replicas of Diftraction Gratinga Moderate in Price and Within Reach of Everyone — Use of Gratings in the Spectroscope— Some Uses of the Spectroscope in Photography — Flame Spectra of V arious Elements — Accurate Work in Color Photography Requires Knowledge of Absorption Spectra (Continued frum last week) FROM our description last week of the manufacture (or ruling) of ditTraction gratings it may have been inferred by the readers that the advantages of spectrum analysis by means of diffraction gratings were restricted to students of large colleges and others of considerable means. Fortunately for science in general, and for the students and workers in color photography in particular, the advantages attending the use of diffraction gratings are available to all, through the medium of the Diffraction Grating Replica. A grating replica is an impression, moulded or otherwise, from an original ruled grating. It will readily be inferred that impressions or casts, from perfect original gratings, may be made and it is found that the making of copies from original gratings does not harm the original, but, under some circumstances may serve to improve the condition of the original by keeping its ruled surface clean. Several workers in the fields of physics and photography have appreciated the importance of placing grating replicas into the Jt. = liands of students, and years of experimenting have resulted in the production of extremely accurate grating replicas or copies. Rayleigh's Elxperiments Lord Rayleigh. the illustrious English physicist, was the first to attempt the manufacture of grating replicas in a practical way (1872). Rayleigh proposed to make photographic copies from his glass transmission gratings by the process of contact printing. He placed a glass grating in a printing frame. Upon this he placed a sensitized collodion plate and exposed the same to direct rays of sunlight. Exposure and development completed, the result was a photographic copy of the original. Rayleigh also tried printing upon plates coated with Bichromated Gelatine and with Bitumen, and although he produced and distributed many replicas giving handsome resolution of the lines in the first and , second order spectra, none of his processes proved certain enough in their results to be commercially practicable. Thomas Thorp of Manchester, England, finally commenced the production of grating replicas by the process which is practised (with slight modifications) to day, and which gives highly efficient copies of an original ruling. Method of Making Celluloid Copies Briefly, Thorp's procedure was to place a reflection grating (speculum metal grating) upon a levelling slab and to pour upon the ruled surface a solution of gun-cotton in amyl-acetate (celluloid— practically speaking). After the film of celluloid had dried it was carefully stripped from the grating under water, the result being a cast or copy (on celluloid) of the ruled surface. This cast being, of course, very thin and fragile, a skin of celluloid was then placed upon an accurately surfaced piece of glass with its ruled surface upwards and sealed to the glass with Canada Balsam. F. E. Ives, an American, well known as the father of the Photo-engraving Process, and as an able exponent of 3-color photography, was the next to try his hand at making replica gratings. From a 15,000 line Rowland original Ives made numerous celluloid casts in a manner similar to that practised by Thorp. Ives practised a new departure in mounting some of his celluloid casts to the glass. After mounting the cast upon one plate of glass Ives proposed to seal another glass plate over the ruled surface, to protect it from possible damage. This proved to be a serious drawback, however, as the sealing compound (in numerous instances) penetrated the celluloid ruling and caused its deterioration and ultimate destruction. It was furthermore pointed out by R. James Wallace that interference bands were likely to be produced by placing a cover glass in contact with the ruling. We will consider interference phenomena further on, but it may be stated here that interference bands are very annoying to encounter in spectroscopic work. Wallace Replicas Best To the last mentioned worker, Mr. R. J. Wallace, who has been mentioned before in the Camera Department, must be accorded supremacy in the manufacture of diffracting grating replicas. Wallace also makes his replicas by flowing celluloid over an original ruling, but, in addition to a great deal of original experimenting, he has taken into account the mistakes of the others and the Wallace replicas are capable of duplicating any performance which the original gratings are capable of. Wallace's copies (mostly from Rowland originals) are to be found in schools, colleges and laboratories throughout the world. They are mounted upon very accurately surfaced glass and are entirely free from any bubbles or other defects incident to the mounting operation. As a first class grating replica with one inch of ruled surface containing 14,000 lines (or upwards) can be secured for $5.00 it is apparent that anyone working with spectroscopes can secure results equal to those obtained with costly rulings, and at a nominal cost. Use of Grating in the Spectroscope All types of gratings generally employed having been described, a few words are now in order as to the manner in which a grating (or grating replica) is used in the spectroscope. In order to successfully utilize a diffraction grating it is necessary to have a spectroscope with the telescope (or viewing tube) pivoted to the center of the stand — so that the telescope can be swung from side to side. If, in addition to the swinging telescope, the spectroscope also has, below the prism table, a circular disc graduated in degrees over which a pointer attached to the telescope travels, it is called a spectrometer. With a spectrometer the deviation of the telescope necessary to locate a given line may be measured and the reading duplicated at any future time. In A, Fig. 5, a transmission grating G (a replica — being transparent— is similar) is shown in place upon the prism stand of (Continued on page 2085)