152 SUMMARY OF CURRENT RESEARCHES RELATING TO 



the concave side of a plano-concave towards the window, the concave side 

 will give an inverted image, and the plane side an erect image ; but with 

 the plane side to the window, you will get two erect images. Converging 

 and diverging menisci have for their convex sides two erect images, and 

 for their concave sides two inverted. I find, however, that in a converging 

 meniscus, if the concave surface is of very large radius, the reflection from 

 it when viewed from the convex side will be inverted instead of erect ; in 

 other words, it will take the form of a plano-convex. I imagine that in a 

 diverging meniscus, which closely approximates the form of a plano-concave, 

 the same result would be found — viz. that the image from the flat side, 

 when seen through the more concave side, would be erect instead of 

 inverted, as one would expect ; but of this I have no practical experience, 

 not having a single lens of that form to experiment on. 



" Now, if we take a cemented doublet, consisting of a biconvex and a 

 plano-concave, we shall very easily see the two bright reflections from the 

 two exterior surfaces — viz. the plane and the convex. The image from the 

 cemented surfaces, however, will not be so readily apparent. With a little 

 attention it will be discovered as a faint image, with most probably a bluish 

 tinge, though occasionally it may have a reddish tinge. When once seen, it 

 will be easily recognized again. A triple combination will have two faint 

 images as well as two bright ones. I find the following the best method of 

 procedure. First find out by the number of faint reflections if the lens 

 is a doublet or a triplet. Next find out the nature of the external sur- 

 faces, and write them down — e. g. plano-convex doublet. This means that 

 the combination is composed of two lenses, and that one of the external 

 surfaces is convex and the other plane. Now write down the reflections as 

 they come, beginning at the side next the window, underlining the reflec- 

 tion from the first surface, and putting the reflection from the cemented 

 surface in (). In writing these down, I use the following abbreviations : 

 e for erect, i for inverted, s for small, I for large, and L for very large. It 

 is a good plan to draw the lens by representing, first, the external surfaces 

 only, and then filling in the cemented surfaces, according to the reflections 

 you obtain. It is absolutely necessary that the reflections from both sides 

 of the combinations should be ascertained, as it is impossible to discover 

 the construction of the combination from one set of reflections. When 

 the images are large it is as well to look at the reflection of the bar across 

 a window ; the knob of the hasp showing if the image is erect or inverted. 

 The images from small lenses require to be examined by a magnifying 

 glass. One word of caution, and that is, until one is practised in picking 

 up these faint images, the very large faint ones are apt to be overlooked. 

 Until one is familiar with the manner of holding a lens, only a faint blue 

 tinge will be seen over the glass ; but after a little practice, a distinct 

 image of the window bar will be obtained." 



Some examples with figs, are given. 



(3) Illuminating Apparatus. 



Ahrens's Polarizing Prism.* — Dr. H. Schroder suggests that this 

 prism I may be improved by using linseed oil for cementing instead of 

 Canada balsam, since the surfaces may then be cut at a more convenient 

 angle. This cement is not very tenacious, so that during the cutting and 

 polishing of the prism the parts must be provisionally fastened with 

 Canada balsam, which is finally removed and replaced by the linseed oil 

 varnish. 



* ZeitBchr. f. Instrumentenk.. vi. (188G) pp. 310-1 (1 fig.), 

 t See this Joarnal, 1886, p. 397. 



