When  we  turn  to  the  sexual  difference  in  regard
to  colour-blindness,  a  subject  which  has  been  very
fully  investigated,  there  is  no  doubt  whatever  about
the  results.  Men  are  much  more  frequently  colour-
blind than  women.  The  committee  on  colour-
blindness of  the  Ophthalmological  Society  found
that  among  males  generally  {i.e.,  out  of  14,846
individuals)  the  percentage  of  pronounced  cases
of  colour-blindness  was  3.5,  the  average  percentage
being  4.16.  They  found  that  colour-blindness  was
nearly  always  much  slighter  in  females  than  in
males,  and  even  then  only  existed  in  0.4  per  cent.^
Holmgren,  from  an  examination  of  32,000  men,
found  3.17  per  cent,  colour-blind.  Dr.  Joy  Jefferies
of  Boston,  from  an  examination  carried  on  chiefly  in
educational  institutions,  of  over  18,000  males,  found

'  J.  A.  Gilbert,  Studies  from  the  Yale  Psych.  Lab.,  vol.  ii.,  1894,
p.  58.
■^  "Report  of  the  Committee  on  Colour-Bliiidness  of  the  Ophthal-
mological Society,"  Trans.  Ophthal.  Soc.,  1881.
THE    SENSES.  153

4.1  per  cent,  colour-blind,  and  among  over  14,000
females  only  0.008  were  colour-blind.  Mr.  T.  H.
Bickerton  finds  the  percentage  0.16.  Therefore
while  colour-blindness  exists  in  about.  30  or  40  per
thousand  males  of  the  general  population  of  European
countries  it  is  found  in  onlj'  i  to  4  per  thousand
females,  being  thus  at  least  ten  times  more  frequent
in  men  than  in  women.  One  woman  to  ten  men  is
the  proportion  found  by  Favre  in  France.^

There  are  certain  variations  in  the  incidence  of
colour-blindness,  among  classes  of  the  population
and  among  races,  which  are  of  interest,  and  may
possibly  bear  on  the  significance  of  colour-blindness.
