in Theory and Practice. 383 



than the negative, is albumenized and then treated with 

 collodio-chloride. It is then put in any developer until it 

 turns black. A longitudinal strip of the width of the nega- 

 tive is scraped off, and on the edge of this strip the scale is 

 ruled with a dividing-engine. The negative is clamped in 

 place to this scale, and together they are put in the enlarging 

 camera. The accuracy with which the scale can be made and 

 the negative fitted to it is most satisfactory. On Professor 

 Rowland's new map the greatest error is "03 of an Angstrom 

 unit, and the probable error is less than "02 of a unit. If the 

 scale is, say, "0001 too large or too small, the photographs 

 can be made to fit the scale by altering the distance between 

 the grating and camera-box by '0001 of its amount, and then 

 focusing by moving the slit in or out. 



When this scale is once made, it can be used to give direct 

 readings for the wave-lengths of the lines on any negative 

 simply by placing the negative on the scale. 



A word should be said as to the difliculties of ruling gra- 

 tings, which may explain why so many orders for gratings 

 remain unfulfilled. It takes months to make a perfect screw for 

 the ruling-engine, but a year may easily be spent in search of 

 a suitable diamond-point. The patience and skill required 

 can be imagined. For the past year all attempts to find a 

 point for the new ruling-engine have failed, and it is only 

 wdthin a few days that one has been found. Most points 

 make more than one " furrow " at a time, thus giving a great 

 deal of diff'used light. Moreover few diamond-points rule 

 with equal ease and accuracy up hill and down. This defect 

 of unequal ruling is especially noticeable in small gratings, 

 which should not be used for accurate work. Again, a gra- 

 ting never gives symmetrical spectra ; and often one or two 

 particular spectra take all the light. This is of course de- 

 sirable if these bright spectra are the ones which are to be 

 used. Generally it is not so. These individual peculiarities 

 of gratings were fully treated by Professor Rowland in his 

 lectures during the spring term of 1888 ; and have been 

 embodied by him in a complete mathematical theory of the 

 grating, which he has nearly ready for publication. It is not 

 easy to tell when a good ruling-point is found ; for a 

 "scratchy" grating is often a good one; and a bright ruling- 

 point always gives a "scratchy" grating. When all goes 

 well, it takes five days and nights to rule a 6-inch grating 

 having 20,000 lines to the inch. Comparatively no difiiculty 

 is found in ruling 14,000 lines to the inch. It is much harder 

 to rule a glass grating than a metallic one ; for to all of the 



