40 REPORT — 1870. 



pseudoscopic and superficial, gave no tnistwortliy information of structure — and 

 nence the disagreements of observers. 



Describing the binocular microscopes preceding his own, he contended that they 

 failed to give true appearances, because the second image was but a distorted reflex 

 of the first, and could not thus give true stereoscopic relief. 



Such relief being due to the direct convergence of the axes of two eyes to one 

 object, he had after some years succeeded in making a binocular microscope capable 

 of showing perspective distance, by employing two equally inclined microscopes of 

 equal power. 



Eight drawings of mechanical arrangements accompanied the paper; and the in- 

 struments on this plan were sho^mi to secure to the microscopist and amateur the 

 following advantages : — 



1. The ease of observation by the equal use of both eyes, and a natural erect view 

 for dissecting-pui-poses, 



2. Perfectly stereoscopic views of opaque objects, and distinct definition of 

 thick transparent ones. 



.3. Pseudoscopic -vision for the illustration and verification of all objects, and 

 convertibility into a monocular microscope by the turn of a milled head. 



On Colour-vision at different points of the Retina. 

 By J. Clekk Maxavell, LL.D., F.R.SS. L. 6f E. 



It has long been known that near that point of the retina where it is intersected 

 by the axis of the eye there is a yellowish spot, the existence of which can be 

 shown not only by the ophthalmoscope, but by its effect on vision. At the Chel- 

 tenham Meeting in 185(3 the author pointed out a method of seeing this spot by 

 looking at that part of a very narrow spectrum which lies near the line F. Since 

 that time the spot has been described by Ilelmholtz and others ; and the author has 

 made a number of experiments, not yet published, in order to determine its eflects 

 on colour-vision. 



One of the simplest methods of seeing the spot was suggested to the author by 

 Prof. Stokes. It consists in looking at a white surface, such as that of a white cloud, 

 through a solution of chloride of chromium made so weak that it appears of a 

 bluish-green colour. If the observer directs his attention to what he sees before 

 him before his eyes have got accustomed to the new tone of colour, he sees a 

 pinkish spot like a wafer on a bluish-green gTOund ; and this spot is always at the 

 place he is looking at. The solution transmits the red end of the spectrmii, and 

 also a portion of bluish-gi-een light near the line F. The latter portion is partially 

 absorbed by the spot, so that the red light has the preponderance. 



Experiments of a more accurate kind were made with an instrument the original 

 conception of which is due to Sir Isaac Newton, and is described in his ' Lectiones 

 Opticc'e,'_though it does not appear to have been actually constructed till the author 

 set it up in 1862, with a solid frame and careful adj ustments. It consists of two parts, 

 side by side. In the first part, white light is dispersed by a prism so as to form a 

 spectrum. _ Certain portions of this spectrum are selected "by being allowed to pass 

 through slits in a screen. These selected portions are made to converge on a second 

 prism, which unites them into a single beam of light, in which state they enter 

 the eye. _ The second part of the instrument consists of an arrangement by which a 

 beam of light from the very same somxe is weakened by two reflections from glass 

 surfaces, and enters the eye alongside of the beam of compound colours. 



The instrument is fonned of three rectangular wooden tubes, the whole length 

 being about nine feet. It contains two prisms, two mirrors, and six lenses, which 

 are so adjusted that, in spite of the very diU'erent treatment to which the two por- 

 tions of a beam of light are subjected, they shall enter the eye so as to form exactly 

 equal and coincident images of the soiu-ce of light. In fact, by looking through 

 the instrunaent a man's face may be distinctly seen by means of the red, the green, 

 or the blue light which it emits, or by any combination of these at pleasure. 



The arrangement of the three slits is made by means of six brass slides, which 

 can be worked with screws outside the instrument; and the breadth of the slits can 

 be read off with a gauge very accurately. 



