146 Observations on Monochromatic Light. 



place, have its axis horizontal ; because in this case the eye is 

 in its natural position, and the fluid which lubricates the cor- 

 nea is more evenly diffused over it by its gravity. If the axis 

 of the eye were vertical, the fluid would tend to accumulate at 

 the centre of the cornea, and produce a less perfect refraction 

 of the incident rays. The direction of the lines in the micro- 

 scopic object must be vertical, or parallel to the lines in which 

 the fluid descends over the cornea, because in this position 

 their sharpness will be least affected by the descent of the 

 fluid. And, lastly, the direction of the lines should be in that 

 of a diameter of the field of view passing through the axis of 

 the lens, because in this position all colour whatever will dis- 

 appear between the lines, as the plane of refraction in the 

 lens is coincident with the lines ; a case in which the largest 

 refracting angle of the most dispersive prism is incapable of 

 producing any colour in a line of light. When the lined ob- 

 ject is placed out of the centre of the field, the prismatic co- 

 lours will immediately appear at the edges of the lines ; but 

 this species of colour may be corrected by the opposite refrac- 

 tion of a prism ; or even by looking past the edges of the fin- 

 ger, so as to produce a compensation by the chromatic effect of 

 the eye. This last resource will of course obstruct a good deal 

 of light, and is mentioned chiefly for the sake of illustration. 



We leave it to the mathematicians to deal with the remain- 

 ing part of Dr Goring's paper, which displays great practical 

 knowledge of optical instruments, and contains much original 

 and valuable information ; — but we cannot avoid touching up- 

 on one topic. Dr Goring has in page 67 quoted a passage 

 from Mr Coddington's preface to his Treatise on the Eye and 

 Optical Instruments, which has no doubt compelled him to take 

 up the gauntlet thus thrown down to every improver of the mi- 

 croscope. The following is the passage referred to, which has 

 given just grounds of offence, not only to those who have suc- 

 cessfully devoted their time and their fortune to the improve- 

 ment of the microscope, but to those also who feel that English 

 science might justly boast of its achievements in this department 

 of optics. While the French and Bavarians have been carrying 

 nff from us the palm in achromatic telescopes, it was some con- 



