Fig. 22. 
OPTICAL IMAGES. 
CHAPTER II. 
31. Experimental verification. — 32. Variation of the magnitude 
of the image.— 33. Principal focus and focal length.— 34. Varia¬ 
tion of position, and magnitude of image.— 35. When images real, 
and when imaginary.— 36. Images produced by concave lenses. 
37. Focal length varies with refracting power.— 38. Refracting power 
depends on material of lens.-— 39. Spherical aberration.— 40. Images 
produced by leuses not absolutely clear and distinct.— 41. Series of 
images.— 42. Nebulous and confused effect.— 43. Spherical aberra¬ 
tion greater near the borders. —44. Increases with the curvature. 
—45. And with the magnifying power.— 46. Spherical distortion.— 
47. Curved images.— 48. How to diminish spherical aberration.— 
49. Lenses made from diamonds and other precious stones.— 50. 
Ineffectual attempts at improvement by this means.— 51. Methods 
of diminishing spherical aberration by proper adaptation of curva¬ 
tures.— 52. Aplanatic lenses.— 53. Chromatic aberration.— 54. White 
light compound.— 55. Coloured lights sometimes compound.— 56. 
Images produced by homogeneous lights.— 57. Images produced by 
compound light.— 58. Lenses always produce several images of a 
natural object.— 59. Why they are not always so confused as to be 
useless for vision.— 60. Dispersion.— 61. Dispersion increases with 
refraction.— 62. Dispersion different with different material. 
31. This remarkable property of lenses, which is of the most 
extreme importance, not only in the theory and practical con¬ 
struction and application of microscopes, hut of all optical 
instruments whatsoever, admits of the easiest and most simple 
experimental verification. 
Lardner’s Museum of Science. h 
No. 82. 
97 
