ZOOLOaY AND BOTANY, MICROSCOPY, ETC. 437 



through the lens at 0. The milled head Sj shifts the slit from side to 

 side so as to bring various portions of the spectrum into the centre of 

 the field, whilst that at s^ opens and shuts the slit. The apparatus is 

 attached to the substage at Spk. With ordinary daylight it is only 

 available with low powers. With direct sunlight (and a heliostat) 

 high powers can be used. 



Physiology of Variable Apparent Mag-nification by the Micro- 

 scope.* — In estimating the size of an object viewed in the Microscope, 

 it is commonly assumed that the image is seen as if at the distance of 

 easiest vision, which is taken to be 10 in. The invalidity of this 

 latter assumption, Mr. W. Le Conte Stevens considers, is strikingly 

 shown in the table of estimates exhibited and discussed by Prof. 

 Brewer in his recent paiDer."}" 



" It is well known that the distance of easiest vision is variable 

 during the life of the same individual. The ' near-point ' for a normal 

 eye varies from 3 in. for a child of three years, to 18 or 20 feet for 

 a man of eighty, the power of accommodation diminishing with in- 

 crease of age. For such an eye, when in a relaxed state, parallel rays 

 will be converged to the exact distance of the retina. If the radiant 

 point be but 10 in. distant, the sheaf of divergent rays from it, if 

 transmitted through the same refracting medium, would be focalized 

 behind the retina, were there not an instant contraction of the ciliary 

 muscle, resulting in an increase of convexity of the crystalline lens at 

 its front surface. The ease or difficulty with which this is done 

 depends mainly on the age of the person, if the eye be normal. The 

 effort exerted by a little child will be far less than that of an old 

 man. 



All that the Microscope can do is to increase the visual angle 

 under which the object is seen, and hence increase the size of the 

 retinal image. The extent to which this may be advisable depends 

 upon several considerations well known to microscopists. Since the 

 visual angle is simply the measure of the difference of direction 

 between two rays passing axially through the crystalline lens, from 

 the opposite marginal points of the magnified image, as seen through 

 the eye-piece, it is quite possible for this to remain sensibly constant, 

 while the refracting power of the crystalline lens varies. The adjust- 

 ment of the eye-piece, or the distance of the eye from it, may vary 

 while distinct vision is retained, the limits of variation depending 

 upon the power of accommodation in the eye of the observer. For a 

 hypermetropic eye, the rays from a given crossing-point near the focus 

 of the eye-piece may emerge from the latter either parallel, or slightly 

 convergent, or divergent, and yet be distinctly focalized on the retina 

 in consequence of appropriate action of the ciliary muscle. 



The interpretation which we put upon a retinal sensation is quite 

 unconscious, and always accompanied with equally unconscious inter- 

 pretations of attendant muscular sensations. The experience of the 

 individual is the only guide in reading visual judgments. It is not 



* Amer. Mon. Micr. Journ., iii. (1882) pp. 189-91. 

 t See this Journal, ii. (1882) p. 861. 



