344 Reply to Mr. Templeton's Remarks on a New Optometer. 



ried to inaccuracy, and Mr. Templeton's objection is perfectly 

 valid. In referring to the eye of an albino &c, "this has the 

 advantage that, as the choroid is destitute of pigment, light can 

 traverse it without loss ;" in other words, the choroid is perfectly 

 transparent. The distance of distinct vision has been retained, 

 although it has been proved such does not exist as a definite 

 distance ; the region of accommodation, near point, and far point 

 are not referred to ; myopia and presbyopia are regarded as op- 

 posite defects ; short sight is defined as " the habitual accommo- 

 dation of the eyes for a distance less than that of ordinary vision;" 

 spectacles are calculated from the distance of distinct vision ; 

 as a proof of chromatic aberration in the eye, it is stated, the 

 image of a white disk on a dark ground, if formed before or 

 behind the retina, appears surrounded by a very narrow blue 

 edge, and the optical centre of the lens is used without reference 

 to the effect of the thickness of the refractive media. Brevity 

 cannot be urged as an excuse for inaccuracy. Dr. Donders's 

 ' Anomalies of Refraction and Accommodation ' was published in 

 English in 1864; and Professor Helmholtz's Physiologische 

 Optik was published early in 1867. 



Carpenter's- f Physiology ' has been brought up to date; this 

 and the recent publication of several surgical works must tend to 

 disseminate views consonant with the more recent discoveries. 

 The work selected by me as an example no doubt does good service 

 in the cause of elementary scientific education ; but I think such 

 works as Professor TyndalPs ' Heat ' and ' Sound/ and the excel- 

 lent philosophical elementary works of Professor Huxley, do more 

 good, and are more useful to the teacher, than works which treat 

 briefly of ten or twelve subjects. Professor Huxley's elementary 

 lectures appear, in my humble opinion, perfect models, not 

 cramming many facts into little space, and fully developing the 

 capacity of elementary scientific instruction for training the 

 mind in correct habits of thought. I do not think such general 

 works as those referred to above could be made to approach such 

 a model without greatly increasing their bulk and making the 

 expense of using them a still greater objection than it is at pre- 

 sent; while the vast labour of reference in compiling a work on 

 so many subjects must preclude the possibility of the facts being 

 so reliable as in more special works. It may be urged that a 

 work treating only of the rudiments of a subject can be easily 

 kept up to date; but there is yet the danger of even this foun- 

 dation being shaken by discovery, and of statements no longer 

 true being copied from time to time, from the difficulty, or even 

 impossibility, of one man being equal to the labour of the neces- 

 sary reference. 



