14 E. M. NELSON ON THE GENERAL CHARACTER OF 



image ; which difference is also true of a concave mirror, viz., 

 the greater the concavity the smaller the image. 



If you now examine a single biconvex lens, you will see a 

 large erect image from the surface next the window, and a 

 small inverted image from the surface on the other side. It 

 acts precisely as if it were a convex and a concave mirror. In 

 a single biconcave lens you have a large inverted, and a small 

 erect image. In a plano-convex, with the convex side towards 

 the window, you will find a small erect image from the convex 

 side, and a large inverted image from the plane side. With the 

 plane side towards the window, you will have a large erect 

 image from the plane side, and a small inverted one from the 

 other side. 



With 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 converg- 

 ing 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, seen through the more concave side, would be erect 

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

 practical experience, as I have not got a 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 combina- 

 tion will have two faint images, as well as two bright ones. I 

 find the following the best method of procedure : — 



First, lind out by the number of faint reflections if the lens 



