OF THE RETINA TO LIGHT AND COLOUR. 
1G3 
The first experiments were made with three illuminated circular discs, 2 inches, ^ inch, 
and I inch in diameter, and reduction necessary to cause invisibility (which in future 
will be called, as it was in “ Colour Photometry,” Part III., the “ extinction”) for 
different parts of the spectrum was noted for both. The angular measure of these 
discs was 4° 11', 1° 3', and 31' respectively. 
The following were the readings obtained. The scale of the spectrum has been 
reduced to the standard scale used in Part III., “ Colour Photometry.” 
V 
Table HI. 
Standard 
2-incb disc readings. 
4-incb disc readings. 
j-incb disc readings. 
j numbers. 
i 
Degrees. 
Logs. 
Degrees. 
Logs. 
Degrees. 
Logs. 
1 56-9.3 
110 
3-05 
40 
3-65 
1 55-82 
125 
2-83 
71 
3-38 
20 
3-83 
! 53-60 
198 
2-30 
139 
2-81 
80 
3-31 
! 51-38 
235 
1-93 
180 
2-45 
128 
2-90 
49-16 
277 
1-62 
220 
2-11 
160 
2-69 
46-94 
310 
1-34 
250 
1-85 
187 
2.36 
44-72 
331 
1-15 
275 
1-63 
215 
2-15 
42-50 
345 
1-05 
290 
1-51 
230 
2 02 
40-28 
.345 
1-05 
285 
1-55 
225 
2-07 
38-06 
340 
1-07 
281 
1-57 
221 
2-09 
' 35-84 
335 
1-12 
275 
1-62 
218 
2-12 
i .33-62 
320 
1-24 
255 
1-81 
195 
2-22 
31-40 
300 
1-42 
240 
1-93 
180 
2-45 
29-18 
280 
1-59 
230 
2-02 
165 
2-58 
26-96 
265 
1-74 
207 
2-22 
1.50 
2-71 
24-74 
240 
1-94 
180 
2-45 
130 
2-88 
' 22-52 
215 
2-15 
154 
2-68 
LIO 
3-05 
1 20-30 
205 
2-24 
145 
2-75 
92 
3-24 
! 18-08 
193 
2-30 
137 
2-81 
80 
3-31 
15-86 
180 
2-45 
115 
3-00 
60 
3-48 
1 1.3-64 
162 
2-61 
96 
3-17 
45 
3-61 
All these readings were taken with the same intensity of light, and are, therefore, 
comparable with one another. These log readings were plotted, and freehand curves 
drawn through the points, and the following table constructed from them, the 
standard curve of extinction being added for comparison. 
