240 THE VOYAGE OF H.M.S. CHALLENGER. 



are concave towards the cavity of the eye, and on these areas the ends of the rods rest ; 

 the rods being seen to be aggregated into corresponding bundles in transverse sections. 

 It is not improbable that these curious expansions of the recipient surface of the eye 

 and its retina are a device for detecting the presence of very small quantities of light, 

 at the expense of all apparatus for forming an image." 



Tristan da Cunha Group. 



On the 15th October, at 3 a.m., the ship proceeded towards Tristan Island, and at 

 daylight the three islands of the group were seen. At 8 a.m. the ship was anchored in 19 

 fathoms in Falmouth Bay, and parties landed, with a view of exploring the island and 

 obtaining observations ; but as the weather looked threatening in the afternoon, and a 

 swell got up, it was considered inadvisable to risk remaining at anchor, and the ship left 

 Falmouth Bay proceeding towards Inaccessible Island, with the view of landing on it 

 should the prognostication of bad weather prove incorrect. This was accomplished, for 

 on the 16th, the wind being light and the sea smooth, exploring parties were landed, 

 and the ship steamed round Inaccessible Island, obtaining soundings and dredgings, and 

 finally anchoring on its northeast side for the night. On the morning of the 17th 

 October the vessel left Inaccessible Island for Nightingale Island, and the day was 

 devoted to fixing its position, and surveying and sounding its coast ; but dusk coming on 

 before a suitable anchorage could be found, the ship remained under weigh during the 

 night of the 17th, and the 18th was devoted to sounding and dredging between 

 Nightingale and Tristan Islands, the vessel finally leaving the group for the Cape of 

 Good Hope at 6 p.m. on the 18th (see Sheet 17). 



During the four days' stay the wind varied from N.W. through W. and S. to S.E., 

 the force never exceeding 5, and being frequently 1. The weather was cloudy and the 

 sea moderate. The temperature of the surface water was on an average from 2° to 3° 

 higher than that of the air, the maximum temperature in the shade registered being 56°, 

 the minimum 46°, and the mean 51°; whilst the mean temperature of the surface water 

 was 53°"6. The air was dry and invigorating, the relative humidity averaging 77. 



The islands known as the Tristan da Cunha group (three in number) were originally 

 discovered by the Portuguese about 1506, who named the largest "Tristan da Cunha" 

 (since contracted to Tristan) Island. , The Dutch appear to have described them in 1643, 

 but M. d'Etchevery, in " L'Etoile du Matin," appears to have been the first to land on 

 them in 1767. He named the western island " Inaccessible," and the southern " Nightin- 

 gale " ; and anchored off, and landed on, both Nightingale and Tristan Islands. Since 

 that date many of H.M. ships, as well as merchant vessels, have touched here ; but it 

 was not until they were visited by Captain Denham, H.M.S. " Herald," in 1852, that their 

 exact geographical position was known ; and even in 1873, the precise position and 



