Remarks on a New Optometer. 343 



Mr. Templeton has given an extract without mentioning his 

 authority. I submit, this should have been given, as readers 

 might probably infer from the heading of Mr. Templeton's paper 

 that the quotation represented my opinion. Mr. Templeton in- 

 sists " that further inquiry should take place before it be assumed 

 that ' the known properties of light afford a complete elucidation 

 of the whole mechanism of vision and the use of every part of 

 the visual apparatus/ " I have never met with the above state- 

 ment in works on vision ; on the contrary, I find the admission 

 very general that knowledge in this direction is very incomplete. 



Helmholtz states that doubts may be justly entertained if the 

 state of t.his young and rising branch of science admits of such 

 treatment, even only preliminary, as the plan of his book de- 

 mands. And in another place, as the attempt to introduce order 

 and connexion must ultimately be made, and as the science must 

 be ultimately freed from contradictions, he has undertaken the 

 task under the conviction that order and connexion, even on an 

 unstable basis, are better than contradiction and disconnexion. 

 He also states that the history of the science would only have an 

 interesfcorresponding to the labour of its compilation if the condi- 

 tion of the science itself were riper than it is at present*. In the 

 body of the same work it will be found stated that much remains 

 to be cleared up authoritatively. I have chosen Helmholtz as 

 the leading work on the subject ; but reference to other writers 

 will prove the general prevalence of the above opinion. Ophthal- 

 mic surgical works will, I think, be found to state the matter 

 differently from Mr. Templeton. 



I have remarked above on the information afforded concerning 

 vision by elementary works on " Natural Philosophy," in stating 

 that such works are not to be regarded as authorities while more 

 special works exist. I would not have it understood I think in- 

 accuracy in such works should be excused on the ground of their 

 general character; I consider such works should have that relia- 

 bility which reference to the latest special works alone can ensure. 

 Accuracy both in fact and expression, and strictness in drawing 

 conclusions, are absolutely necessary in works used as much in 

 forming habits of thought as in communicating facts. I regret 

 to say these conditions are not always fulfilled in the general 

 works referred to in treating of vision. For example, in the 

 third edition of a text-book on Physics, a translation from the 

 French, dated July 1868, in the chapter on the Eye considered 

 as an Optical Instrument, I find the following statements : the 

 eye is compared to a camera obscura, " of which the pupil is the 

 aperture, the crystalline the condensing lens, and the retina the 

 screen." In this extract the analogy to a camera obscura is car- 

 * Preface to Physiologische Optik. 



