Micro-ruling on Glass and Steel. 153 



that in addition to being an object of great beauty it throws light 

 upon causes of deceptive appearances, which may usefully warn us 

 against errors of interpretation. 



A simpler glass ruling shows the tendency of the eye to be best 

 satisfied with such a mode of focussing and illuminating intersect- 

 ing lines as gives the aspect of one set being above or below the 

 other. When the lines are smootldy cut, there is also a tendency 

 to prefer that mode of viewing them which gives the aspect of solid 

 threads raised above the surface. The smoothest cuts preserve this 

 character, more or less, with high powers ; and as I have shown in 

 a former paper, the very smooth cracks of sihca films are exceed- 

 ingly deceptive. 



In a glass star of another description, I find bands of converging 

 lines in alternate sets with parallel lines. There are 12 bands 

 of 20 parallel lines each, in each of 10 rays, besides 25 radial hues. 

 " Thus," as Mr. Stanistreet says, " the intersections are extremely 

 numerous." The twelve bands of parallel lines prevent the appear- 

 ance, described in the first star, of a number of spokes of a fan 

 composed of numerous silver wires, and arranged more or less 

 vertically. These fan-spokes are crossed by the radial Knes, and 

 with dark-ground illumination it is easy to show the latter as dis- 

 tinctly overlying the fan-spoke lines. It is also easy to get an 

 opposite appearance in some parts of the same field, and to see the 

 fan-spokes raised above the radial lines. It should be mentioned 

 that the radial lines are far apart as compared with the close bands 

 that form the fan-spokes. If this object were a great deal smaller, 

 and nothing known of its real structure, the diJB&culty of interpret- 

 ing the optical appearances would be great. 



In a star composed entirely of closely radial lines, those which 

 catch the most brilliant light appear to stand above the rest. 



Having suggested to Mr. Stanistreet that beautiful and curious 

 effects might be expected from applying his ruling apparatus and 

 remarkable skill to steel, I soon received from him an exquisite star, 

 much like the one first mentioned, on glass. This star is composed 

 of " 50 radial bands of 40 parallel lines in each band ;" the general 

 pattern being like Fig. 1. 



Held in a bright hght this star exhibits a very elegant appearance 

 of watered silk, with delicate prismatic tints. Under the microscope, 

 with 3 or 4 inch power and illumination with a silver reflector, the 

 star appears as if suspended in a dark atmosphere, or in a bright 

 one, according to the angle at which the light strikes the bright por- 

 tions of the steel. When these portions throw the light they receive, 

 out of the field, the former is the case, and the latter when they send 

 it to the eye. When nicely illuminated each ray glteams with delicate 

 iridescent tints, and the secondary star produced by the intersecting 

 lines can be made to look distinct from the primary star, or as if it 



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