Observer and uninitialized constant

Hello,

I have an application running on rails 1.2.3
I use act_as_authenticated and i want to add an observer on the users
table.

my environment.rb file :

config.active_record.observers = :user_observer

I have created an user_observer.rb file in the model folder like this :
class UserObserver < ActiveRecord::Observer

  def after_save(user)
    Notifications.deliver_user("*****@******.com", "Modification on the
account", user)
  end

end

When i update a user entry, i have a crash and the following message in
production.log file :

  NameError (uninitialized constant UserObserver::Notifications):
      /usr/local/lib/ruby/gems/1.8/gems/activesupport-1.4.2/lib/active_support/dependencies.rb:477:in
`const_missing'
      /app/models/user_observer.rb:5:in `after_save'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/observer.rb:154:in
`send'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/observer.rb:154:in
`update'
      /usr/local/lib/ruby/1.8/observer.rb:185:in `notify_observers'
      /usr/local/lib/ruby/1.8/observer.rb:184:in `each'
      /usr/local/lib/ruby/1.8/observer.rb:184:in `notify_observers'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/callbacks.rb:364:in
`notify'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/callbacks.rb:328:in
`callback'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/callbacks.rb:243:in
`create_or_update'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/base.rb:1545:in
`save_without_validation'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/validations.rb:752:in
`save_without_transactions'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/transactions.rb:129:in
`save'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/connection_adapters/abstract/database_statements.rb:59:in
`transaction'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/transactions.rb:95:in
`transaction'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/transactions.rb:121:in
`transaction'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/transactions.rb:129:in
`save'
      /usr/local/lib/ruby/gems/1.8/gems/activerecord-1.15.3/lib/active_record/base.rb:1592:in
`update_attributes'
      /app/controllers/account_controller.rb:79:in `update'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/base.rb:1095:in
`send'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/base.rb:1095:in
`perform_action_without_filters'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:632:in
`call_filter'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:638:in
`call_filter'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:438:in
`call'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:637:in
`call_filter'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:638:in
`call_filter'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:438:in
`call'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:637:in
`call_filter'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:619:in
`perform_action_without_benchmark'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/benchmarking.rb:66:in
`perform_action_without_rescue'
      /usr/local/lib/ruby/1.8/benchmark.rb:293:in `measure'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/benchmarking.rb:66:in
`perform_action_without_rescue'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/rescue.rb:83:in
`perform_action'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/base.rb:430:in
`send'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/base.rb:430:in
`process_without_filters'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/filters.rb:624:in
`process_without_session_management_support'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/session_management.rb:114:in
`process'
      /usr/local/lib/ruby/gems/1.8/gems/actionpack-1.13.3/lib/action_controller/base.rb:330:in
`process'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/dispatcher.rb:41:in
`dispatch'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:168:in
`process_request'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:143:in
`process_each_request!'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:109:in
`with_signal_handler'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:142:in
`process_each_request!'
      /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.7/lib/fcgi.rb:612:in
`each_cgi'
      /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.7/lib/fcgi.rb:609:in
`each'
      /usr/local/lib/ruby/gems/1.8/gems/fcgi-0.8.7/lib/fcgi.rb:609:in
`each_cgi'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:141:in
`process_each_request!'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:55:in
`process!'
      /usr/local/lib/ruby/gems/1.8/gems/rails-1.2.3/lib/fcgi_handler.rb:25:in
`process!'
      dispatch.fcgi:24

Have you an idea ?
Thanks,