318 



CATALOGUE. — PHySICAl OPTICS. 



may be rendered a verjr beautiful object by means of the 

 solar microscope. 



The most perfectly plane glasses are those which are used 

 for Hadlcy's quadrants : one of these may be ground in the 

 direction of the diagonal of its transverse section, so as to 

 make a thin wedge or prism ; and the surface of the lens 

 employed must be a portion of a sphere of from five to ten 

 ■feet radius. The two glasses must be retained in their po- 

 sition by means of three screws ; for, as soon as the pressure 

 is remoTcd, they repel each other with considerable force ; 

 and, for this reason, neither of them ought to be very thin, 

 otherwise they will bend before they are sufficiently near. 



For adjusting the glasses of the microscope, it is conve- 

 nient to fix them in a cylinder of sufficient size to project 

 beyond the glasses and their screws, in order that they may 

 be readily turned so as to reflect the light coming from the 

 speculum, into the direction of the axis of the microscope : 

 it is obvious, that in this case, they must be somewhat in- 

 clined to the light, so that the focus of the whole image 

 will never be equally perfect ; and, instead of being circular, 

 like the rings themselves, their images on the screen will 

 be oval. In this manner, eight or ten alternations of co- 

 lours may easily be observed ; but their order and sequence 

 is too complicated to be easily understood ; for they are 

 really composed of an infinite number of series of rings of 

 different magnitude, each series being formed by each of the 

 gradations of light in the prismatic spectram, which, near 

 the centre, are sufficiently separate to form distinct appear- 

 ances, either alone or in combination ; but, after eight or 

 ten alternations, are lost in the common effect of white 

 light. For, when the glasses are illuminated by homoge- 

 neous light only, separated from the rest by the refraction 

 of a prism, or otherwise, the rings of each colour occupy, 

 together with the dark spaces, the whole visible surface, 

 their number being only limited by the power of the eye 

 ,in perceiving objects so minute as the external ones be- 

 come, in consequence of the rapid increase of the thick- 

 ness of the plate of air near the edges of the curved surface. 

 This circumstance being once understood, it is also capable 

 of being illustrated in a manner still more elegant, by 

 placing a prism a few feet from the microscope, leaving 

 only a narrow line of its surface exposed to the incident rays, 

 and then throwing the rings of colours on it, in such a direc- 

 tion, that this line shall pass through their centre. Care 

 being taken to exclude from the prismatic spectrum thus 

 formed all extraneous light, it exhibits a most interesting 

 analysis of these colours ; for the line consists of portions 

 of the rings of all possible gradations of colour, each form- 

 ing a broken line, but not of the same dimensions ; and, 

 by tjie prismatic refraction, all these broken lines are sepa- 



rated and placed parallel to each other, on account of th« 

 different refrangibility of the light of which they consist. 

 Thus the broken line of the eiueme red, which consists of 

 the longest portions, is least refracted ; the other reds fol- ' 

 low, and are placed in contact with the first, and with each 

 other, but, on account of the different magnitude of the 

 portions, somewhat obliquely. The dark spaces also are in 

 contact, and form a separation between each portion of 

 light. In the same manner, the green follows the red, 

 with little or no visible yellow. The blue and violet are , 

 somewhat mixed : for these two colours are much less 

 widely separated by thin plates than by the prism : for this 

 reason, each portion of light formed by the contiguous lines 

 of the different colours is bounded not by straight but by 

 curved lines. 



It is evident, that, by drawing a line across this compound 

 spectrum at any part, we may learn the component parts 

 of the light constituting the rings at that part ; for the prism 

 only spreads the colours in a direction transverse to this 

 spectrum : and it may be observed, that after tlie eighth or 

 tenth alternation, the light transmitted at each point is so 

 mixed, that we may easily understand how it appears white. 



The colours of thin plates, as seen by transmission, are 

 also easily exhibited in the solar microscope ; but, since it 

 is utterly impossible to exclude the very great proportion of 

 the light which does not appear to be concerned in their 

 formation, they are never so brilliant as the colours seefi by 

 reflection. 



Account of Dr. Young's Experiments and Calculations 

 relative to Physical Optics. From the Journals of the 

 Royal Institution, II. 



Dr. Young divides this paper into six sections. 1 . Ex- 

 perimental demonstration of the general law of the inter- 

 ference of light. 2. Comparison of measures, deduced from 

 various experiments. 3. Application to the supernumerary 

 rainbows. 4. Argumentative inference respecting the na- 

 ture of light. 5. Remarks on the colours of natiual bodies, 

 6. Experiment on the dark rays of Ritter. 



The object of the first section is to demonstrate in a sim- 

 ple and elementary manner, by the direct evidence of the 

 senses, the truth of the genera! principle, which appears to 

 connect an extensive class of phenomena by a clear analogy. 

 This principle is, that where two portions of light arrive at 

 any point by different routes very nearly in the same di- 

 rection, they sometimes destroy and sometimes corroborate 

 each other, according to the different lengths of their re- 

 spective paths. This is proved by placing a slip of card in 

 a sun beam admitted through a small aperture, its shadow 

 being divided by alternate lines of light and shade w^ep 



