12 GORHAM, ON THE 



in balsam. In this way they were rendered sufficiently 

 diaphanous to transmit a strong light from the sun without 

 injuring the eye, and their reticulations were not obscured 



Having prepared the object, it was necessary to decide upon 

 the distance from the eye best suited for examining it; and 

 as the nearest position would insure the largest image, this 

 was accordingly adopted. The mode of illumination also 

 required consideration, in order that the light should be of 

 such intensity as to make the object visible without dazzling 

 or injuring the eye. The quantity of liglit received by any 

 object is often measured with an instrument invented for the 

 purpose, called a photometer ; but a box with small apertures 

 pierced through the bottom might be shown to constitute a 

 good and efficient substitute for such an instrument. For it 

 is to be inferred from the remarks contained in the sixth para- 

 graph, that when a small object is examined by the aid of the 

 pencils of light admitted through such apertures, it is illumi- 

 nated by the sum of their intensities. Hence the quantity of 

 light thrown upon the object will be regulated by the number 

 of the apertures, being very nearly proportional to them : that 

 is to say, the intensity from two apertures will be nearly twice 

 as great as that from one. 



By varying the number of apertures, therefore, we can regu- 

 late the illumination of the object with the greatest nicety. 

 It follows that there is no better way, perhaps, of exploring 

 minute objects with the naked eye, than by holding them, 

 mounted on a slide of glass, as near to the eye as possible, and 

 examining them by looking at them from the inside of a small 

 box through apertures made in the bottom, and which are 

 covered with tracing-paper, by the aid of a strong light. 



And this is effected by using the little instrument, to which 

 I have ventured to give the name of diascope. 



Thus all the conditions were fulfilled ; for, 1, the object 

 was rendered sufficiently transparent to transmit the rays of 

 light freely, and sufficiently opaque to prevent the solar rays 

 dazzling the eye ; 2, it was coloured to make all its parts visible ; 

 3, it was brought sufficiently near to the eye to be enormously 

 magnified ; and 4, all extraneous rays of light, those which 

 were not immediately concerned in the illumination of the 

 object, were shut out. It remains to be noticed that the ex- 

 periment was crowned with success ; for, on examining the 

 membrane by the direct rays of the noonday sun, the whole 

 of its area appeared reticulated, and several well-defined 

 hexagons icere seen in its centre. While, as a red tint had been 

 communicated to the specimen, its reticulations were most 

 easily discerned in the red discs, inasmuch as the tints of all 



